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* Ready to ship.Eric S. Raymond1997-01-241-2/+2
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* Better notification on authentication failure.Eric S. Raymond1997-01-241-0/+5
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* Avoid random return value.Eric S. Raymond1997-01-241-6/+8
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* Correct the interface parsing.Eric S. Raymond1997-01-244-13/+7
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* Clarification.Eric S. Raymond1997-01-241-1/+2
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* Bump version number.Eric S. Raymond1997-01-241-1/+1
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* Better error tracking.Eric S. Raymond1997-01-241-3/+3
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* Introduce gen_recv.Eric S. Raymond1997-01-245-68/+41
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* Set peek_capable properly.Eric S. Raymond1997-01-231-0/+2
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* Make this aware of RFC2060.Eric S. Raymond1997-01-232-11/+66
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* Added RFC2060.Eric S. Raymond1997-01-231-1/+1
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* Updated.Eric S. Raymond1997-01-231-5/+5
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* Comment fix.Eric S. Raymond1997-01-231-1/+1
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* Add notes on RFC1123 conformance.Eric S. Raymond1997-01-231-1/+32
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* Break down features by major release.Eric S. Raymond1997-01-221-19/+22
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* Remove obsolete caveat.Eric S. Raymond1997-01-222-5/+1
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* General ESMTP support seems pretty good now.Eric S. Raymond1997-01-223-16/+22
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* ESMTP SIZE option support.Eric S. Raymond1997-01-225-47/+111
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* Support for EHLO and 8BITMIME extension.Eric S. Raymond1997-01-226-11/+120
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* Portability fix.Eric S. Raymond1997-01-221-4/+4
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* Bump the contact-list count.Eric S. Raymond1997-01-221-1/+1
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* Handle zero-length messages.Eric S. Raymond1997-01-212-4/+18
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* Ready to ship.Eric S. Raymond1997-01-211-2/+2
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* Update beta list number.Eric S. Raymond1997-01-211-1/+1
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* Improved rewrite.Eric S. Raymond1997-01-211-8/+16
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* Speedup advice.Eric S. Raymond1997-01-211-22/+53
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* Update the copy.Eric S. Raymond1997-01-201-3/+5
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* RPOP support is back.Eric S. Raymond1997-01-201-1/+2
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* We're 3.0 now.Eric S. Raymond1997-01-201-1/+1
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* RPOP support is back.Eric S. Raymond1997-01-206-4/+49
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* Make the syntax of name mappings a little nicer.Eric S. Raymond1997-01-201-1/+1
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* Error message correction.Eric S. Raymond1997-01-191-2/+5
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* Avoid core dump again.Eric S. Raymond1997-01-191-0/+1
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* Avoid core dump in rewrite logic.Eric S. Raymond1997-01-191-1/+3
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* George Sipe's monitor fixes.Eric S. Raymond1997-01-193-47/+74
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* Added dns/nodns.Eric S. Raymond1997-01-189-52/+105
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* Fix message-delimiter logic.Eric S. Raymond1997-01-171-2/+1
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* Ready to ship to George Sipe.Eric S. Raymond1997-01-172-2/+2
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* Fix the second incanation of the HELO bug.Eric S. Raymond1997-01-171-5/+6
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* Philippe de Muyter's Motorola fixes.Eric S. Raymond1997-01-168-14/+23
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* Feature clarification.Eric S. Raymond1997-01-161-4/+4
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* New feature announcement.Eric S. Raymond1997-01-161-0/+2
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* This version seems to work OK.Eric S. Raymond1997-01-161-71/+82
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* Simplification time.Eric S. Raymond1997-01-161-7/+3
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* We don't modify lines as they pass through any more.Eric S. Raymond1997-01-161-57/+1
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* We deal with contibuations more directly now.Eric S. Raymond1997-01-161-12/+0
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* One step towards proper continuation handling.Eric S. Raymond1997-01-163-4/+28
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* This should handle RFC822 continuations now.Eric S. Raymond1997-01-161-12/+24
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* This does a reasonable job.Eric S. Raymond1997-01-151-12/+16
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* About ready to ship.Eric S. Raymond1997-01-151-1/+10
| | | | svn path=/trunk/; revision=771
mmon logic and its interface to the rest of the program, both the common and protocol-specific parts become easier to understand.<P> Furthermore, many kinds of new features can instantly be supported across all protocols by modifying the one driver module.<P> <H1>Implications of smtp forwarding</H1> The direction of the project changed radically when Harry Hochheiser sent me his scratch code for forwarding fetched mail to the SMTP port. I realized almost immediately that a reliable implementation of this feature would make all the other delivery modes obsolete.<P> Why mess with all the complexity of configuring an MDA or setting up lock-and-append on a mailbox when port 25 is guaranteed to be there on any platform with TCP/IP support in the first place? Especially when this means retrieved mail is guaranteed to look like normal sender- initiated SMTP mail, which is really what we want anyway.<P> Clearly, the right thing to do was (1) hack SMTP forwarding support into the generic driver, (2) make it the default mode, and (3) eventually throw out all the other delivery modes. <P> I hesitated over step 3 for some time, fearing to upset long-time popclient users dependent on the alternate delivery mechanisms. In theory, they could immediately switch to .forward files or their non-sendmail equivalents to get the same effects. In practice the transition might have been messy.<P> But when I did it (see the NEWS note on the great options massacre) the benefits proved huge. The cruftiest parts of the driver code vanished. Configuration got radically simpler -- no more grovelling around for the system MDA and user's mailbox, no more worries about whether the underlying OS supports file locking.<P> Also, the only way to lose mail vanished. If you specified localfolder and the disk got full, your mail got lost. This can't happen with SMTP forwarding because your SMTP listener won't return OK unless the message can be spooled or processed.<P> Also, performance improved (though not so you'd notice it in a single run). Another not insignificant benefit of this change was that the manual page got a lot simpler.<P> Later, I had to bring --mda back in order to allow handling of some obscure situations involving dynamic SLIP. But I found a much simpler way to do it.<P> The moral? Don't hesitate to throw away superannuated features when you can do it without loss of effectiveness. I tanked a couple I'd added myself and have no regrets at all. As Saint-Exupery said, "Perfection [in design] is achieved not when there is nothing more to add, but rather when there is nothing more to take away." This program isn't perfect, but it's trying.<P> <H1>The most-requested features that I will never add, and why not:</H1> <H2>Password encryption in .fetchmailrc</H2> The reason there's no facility to store passwords encrypted in the .fetchmailrc file is because this doesn't actually add protection.<P> Anyone who's acquired the 0600 permissions needed to read your .fetchmailrc file will be able to run fetchmail as you anyway -- and if it's your password they're after, they'd be able to rip the necessary decoder out of the fetchmail code itself to get it.<P> All .fetchmailrc encryption would do is give a false sense of security to people who don't think very hard.<P> <H2>Truly concurrent queries to multiple hosts</H2> Occasionally I get a request for this on "efficiency" grounds. These people aren't thinking either. True concurrency would do nothing to lessen fetchmail's total IP volume. The best it could possibly do is change the usage profile to shorten the duration of the active part of a poll cycle at the cost of increasing its demand on IP volume per unit time.<P> If one could thread the protocol code so that fetchmail didn't block on waiting for a protocol response, but rather switched to trying to process another host query, one might get an efficiency gain (close to constant loading at the single-host level).<P> Fortunately, I've only seldom seen a server that incurred significant wait time on an individual response. I judge the gain from this not worth the hideous complexity increase it would require in the code.<P> <H2>Multiple concurrent instances of fetchmail</H2> Fetchmail locking is on a per-invoking-user because finer-grained locks would be really hard to implement in a portable way. The problem is that you don't want two fetchmails querying the same site for the same remote user at the same time.<P> To handle this optimally, multiple fetchmails would have to associate a system-wide semaphore with each active pair of a remote user and host canonical address. A fetchmail would have to block until getting this semaphore at the start of a query, and release it at the end of a query.<P> This would be way too complicated to do just for an "it might be nice" feature. Instead, you can run a single root fetchmail polling for multiple users in either single-drop or multidrop mode.<P> The fundamental problem here is how an instance of fetchmail polling host foo can assert that it's doing so in a way visible to all other fetchmails. System V semaphores would be ideal for this purpose, but they're not portable.<P> I've thought about this a lot and roughed up several designs. All are complicated and fragile, with a bunch of the standard problems (what happens if a fetchmail aborts before clearing its semaphore, and how do we recover reliably?).<P> I'm just not satisfied that there's enough functional gain here to pay for the large increase in complexity that adding these semaphores would entail.<P> <H1>Multidrop and alias handling</H1> I decided to add the multidrop support partly because some users were clamoring for it, but mostly because I thought it would shake bugs out of the single-drop code by forcing me to deal with addressing in full generality. And so it proved.<P> There are two important aspects of the features for handling multiple-drop aliases and mailing lists which future hackers should be careful to preserve.<P> <OL> <LI> The logic path for single-recipient mailboxes doesn't involve header parsing or DNS lookups at all. This is important -- it means the code for the most common case can be much simpler and more robust.<P> <LI> The multidrop handing does <EM>not</EM> rely on doing the equivalent of passing the message to sendmail -oem -t. Instead, it explicitly mines members of a specified set of local usernames out of the header.<P> <LI> We do <EM>not</EM> attempt delivery to multidrop mailboxes in the presence of DNS errors. Before each multidrop poll we probe DNS to see if we have a nameserver handy. If not, the poll is skipped. If DNS crashes during a poll, the error return from the next nameserver lookup aborts message delivery and ends the poll. The daemon mode will then quietly spin until DNS comes up again, at which point it will resume delivering mail. </OL> When I designed this support, I was terrified of doing anything that could conceivably cause a mail loop (you should be too). That's why the code as written can only append <EM>local</EM> names (never @-addresses) to the recipients list.<P> The code in mxget.c is nasty, no two ways about it. But it's utterly necessary, there are a lot of MX pointers out there. It really ought to be a (documented!) entry point in the bind library.<P> <H1>DNS error handling</H1> Fetchmail's behavior on DNS errors is to suppress forwarding and deletion of the individual message that each occurs in, leaving it queued on the server for retrieval on a subsequent poll. The assumption is that DNS errors are transient, due to temporary server outages.<P> Unfortunately this means that if a DNS error is permanent a message can be perpetually stuck in the server mailbox. We've had a couple bug reports of this kind due to subtle RFC822 parsing errors in the fetchmail code that resulted in impossible things getting passed to the DNS lookup routines.<P> Alternative ways to handle the problem: ignore DNS errors (treating them as a non-match on the mailserver domain), or forward messages with errors to fetchmail's invoking user in addition to any other recipients. These would fit an assumption that DNS lookup errors are likely to be permanent problems associated with an address.<P> <H1>IPv6 and IPSEC</H1> The IPv6 support patches are really more protocol-family independence patches. Because of this, in most places, "ports" (numbers) have been replaced with "services" (strings, that may be digits). This allows us to run with certain protocols that use strings as "service names" where we in the IP world think of port numbers. Someday we'll plumb strings all over and then, if inet6 is not enabled, do a getservbyname() down in SocketOpen. The IPv6 support patches use getaddrinfo(), which is a POSIX p1003.1g mandated function. So, in the not too distant future, we'll zap the ifdefs and just let autoconf check for getaddrinfo. IPv6 support comes pretty much automatically once you have protocol family independence.<P> <H1>Internationalization</H1> Internationalization is handled using GNU gettext (see the file ABOUT_NLS in the source distribution). This places some minor constraints on the code.<P> Strings that must be subject to translation should be wrapped with _() or N_() -- the former in functuib arguments, the latter in static initializers and other non-function-argument contexts.<p> <H1>Checklist for Adding Options</H1> Adding a control option is not complicated in principle, but there are a lot of fiddly details in the process. You'll need to do the following minimum steps. <UL> <LI>Add a field to represent the control in <code>struct run</code>, <code>struct query</code>, or <code>struct hostdata</code>. <LI>Go to <code>rcfile_y.y</code>. Add the token to the grammar. Don't forget the <code>%token</code> declaration. <LI>Pick an actual string to declare the option in the .fetchmailrc file. Add the token to <code>rcfile_l</code>. <LI>Pick a long-form option name, and a one-letter short option if any are left. Go to <code>options.c</code>. Pick a new <code>LA_</code> value. Hack the <code>longoptions</code> table to set up the association. Hack the big switch statement to set the option. Hack the `?' message to describe it. <LI>If the default is nonzero, set it in <code>def_opts</code> near the top of <code>load_params</code> in <code>fetchmail.c</code>. <LI>Add code to dump the option value in <code>fetchmail.c:dump_params</code>. <LI>Add proper <code>FLAG_MERGE</code> actions in fetchmail.c's optmerge() function. (If it's a global option, add an override at the end of load_params.) <LI>Document the option in fetchmail.man. This will require at least two changes; one to the collected table of options, and one full text description of the option. <LI>Add the new token and a brief description to the header comment of sample.rcfile. <LI>Hack fetchmailconf to configure it. Bump the fetchmailconf version. <LI>Hack conf.c to dump the option so we won't have a version-skew problem. <LI>Add an entry to NEWS. <LI>If the option implements a new feature, add a note to the feature list. </UL> There may be other things you have to do in the way of logic, of course.<P> Before you implement an option, though, think hard. Is there any way to make fetchmail automatically detect the circumstances under which it should change its behavior? If so, don't write an option. Just do the check!<p> <H1>Lessons learned</H1> <H3>1. Server-side state is essential</H3> The person(s) responsible for removing LAST from POP3 deserve to suffer. Without it, a client has no way to know which messages in a box have been read by other means, such as an MUA running on the server.<P> The POP3 UID feature described in RFC1725 to replace LAST is insufficient. The only problem it solves is tracking which messages have been read <EM>by this client</EM> -- and even that requires tricky, fragile implementation.<P> The underlying lesson is that maintaining accessible server-side `seen' state bits associated with Status headers is indispensible in a Unix/RFC822 mail server protocol. IMAP gets this right.<P> <H3>2. Readable text protocol transactions are a Good Thing</H3> A nice thing about the general class of text-based protocols that SMTP, POP2, POP3, and IMAP belongs to is that client/server transactions are easy to watch and transaction code correspondingly easy to debug. Given a decent layer of socket utility functions (which Carl provided) it's easy to write protocol engines and not hard to show that they're working correctly.<P> This is an advantage not to be despised! Because of it, this project has been interesting and fun -- no serious or persistent bugs, no long hours spent looking for subtle pathologies.<P> <H3>3. IMAP is a Good Thing.</H3> If there were a standard IMAP equivalent of the POP3 APOP validation, POP3 would be completely obsolete.<P> <H3>4. SMTP is the Right Thing</H3> In retrospect it seems clear that this program (and others like it) should have been designed to forward via SMTP from the beginning. This lesson may be applicable to other Unix programs that now call the local MDA/MTA as a program.<P> <H3>5. Syntactic noise can be your friend</H3> The optional `noise' keywords in the rc file syntax started out as a late-night experiment. The English-like syntax they allow is considerably more readable than the traditional terse keyword-value pairs you get when you strip them all out. I think there may be a wider lesson here.<P> <H1>Motivation and validation</H1> It is truly written: the best hacks start out as personal solutions to the author's everyday problems, and spread because the problem turns out to be typical for a large class of users. So it was with Carl Harris and the ancestral popclient, and so with me and fetchmail.<P> It's gratifying that fetchmail has become so popular. Until just before 1.9 I was designing strictly to my own taste. The multi-drop mailbox support and the new --limit option were the first features to go in that I didn't need myself.<P> By 1.9, four months after I started hacking on popclient and a month after the first fetchmail release, there were literally a hundred people on the fetchmail-friends contact list. That's pretty powerful motivation. And they were a good crowd, too, sending fixes and intelligent bug reports in volume. A user population like that is a gift from the gods, and this is my expression of gratitude.<P> The beta testers didn't know it at the time, but they were also the subjects of a sociological experiment. The results are described in my paper, <A HREF="//www.tuxedo.org/~esr/writings/cathedral-bazaar/">The Cathedral And The Bazaar</A>.<P> <H1>Credits</H1> Special thanks go to Carl Harris, who built a good solid code base and then tolerated me hacking it out of recognition. And to Harry Hochheiser, who gave me the idea of the SMTP-forwarding delivery mode.<P> Other significant contributors to the code have included Dave Bodenstab (error.c code and --syslog), George Sipe (--monitor and --interface), Gordon Matzigkeit (netrc.c), Al Longyear (UIDL support), Chris Hanson (Kerberos V4 support), and Craig Metz (OPIE, IPv6, IPSEC).<P> <H1>Conclusion</H1> At this point, the fetchmail code appears to be pretty stable. It will probably undergo substantial change only if and when support for a new retrieval protocol or authentication method is added.<P> <H1>Relevant RFCS</H1> Not all of these describe standards explicitly used in fetchmail, but they all shaped the design in one way or another.<P> <DL> <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc821.txt">RFC821</A> <DD> SMTP protocol <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc822.txt">RFC822</A> <DD> Mail header format <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc937.txt">RFC937</A> <DD> Post Office Protocol - Version 2 <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc974.txt">RFC974</A> <DD> MX routing <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc976.txt">RFC976</A> <DD> UUCP mail format <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc1081.txt">RFC1081</A> <DD> Post Office Protocol - Version 3 <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc1123.txt">RFC1123</A> <DD> Host requirements (modifies 821, 822, and 974) <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc1176.txt">RFC1176</A> <DD> Interactive Mail Access Protocol - Version 2 <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc1203.txt">RFC1203</A> <DD> Interactive Mail Access Protocol - Version 3 <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc1225.txt">RFC1225</A> <DD> Post Office Protocol - Version 3 <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc1344.txt">RFC1344</A> <DD> Implications of MIME for Internet Mail Gateways <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc1413.txt">RFC1413</A> <DD> Identification server <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc1428.txt">RFC1428</A> <DD> Transition of Internet Mail from Just-Send-8 to 8-bit SMTP/MIME <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc1460.txt">RFC1460</A> <DD> Post Office Protocol - Version 3 <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc1521.txt">RFC1521</A> <DD> MIME: Multipurpose Internet Mail Extensions <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc1869.txt">RFC1869</A> <DD> SMTP Service Extensions (ESMTP spec) <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc1652.txt">RFC1652</A> <DD> SMTP Service Extension for 8bit-MIMEtransport <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc1725.txt">RFC1725</A> <DD> Post Office Protocol - Version 3 <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc1730.txt">RFC1730</A> <DD> Interactive Mail Access Protocol - Version 4 <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc1731.txt">RFC1731</A> <DD> IMAP4 Authentication Mechanisms <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc1732.txt">RFC1732</A> <DD> IMAP4 Compatibility With IMAP2 And IMAP2bis <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc1734.txt">RFC1734</A> <DD> POP3 AUTHentication command <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc1870.txt">RFC1870</A> <DD> SMTP Service Extension for Message Size Declaration <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc1891.txt">RFC1891</A> <DD> SMTP Service Extension for Delivery Status Notifications <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc1892.txt">RFC1892</A> <DD> The Multipart/Report Content Type for the Reporting of Mail System Administrative Messages <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc1894.txt">RFC1894</A> <DD>An Extensible Message Format for Delivery Status Notifications <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc1893.txt">RFC1893</A> <DD> Enhanced Mail System Status Codes <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc1894.txt">RFC1894</A> <DD> An Extensible Message Format for Delivery Status Notifications <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc1938.txt">RFC1938</A> <DD> A One-Time Password System <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc1939.txt">RFC1939</A> <DD> Post Office Protocol - Version 3 <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc1957.txt">RFC1957</A> <DD> Some Observations on Implementations of the Post Office Protocol (POP3) <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc1985.txt">RFC1985</A> <DD> SMTP Service Extension for Remote Message Queue Starting <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc2033.txt">RFC2033</A> <DD> Local Mail Transfer Protocol <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc2060.txt">RFC2060</A> <DD> Internet Message Access Protocol - Version 4rev1 <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc2061.txt">RFC2061</A> <DD> IMAP4 Compatibility With IMAP2bis <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc2062.txt">RFC2062</A> <DD> Internet Message Access Protocol - Obsolete Syntax <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc2195.txt">RFC2195</A> <DD> IMAP/POP AUTHorize Extension for Simple Challenge/Response <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc2177.txt">RFC2177</A> <DD> IMAP IDLE command <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc2449.txt">RFC2449</A> <DD> POP3 Extension Mechanism <DT><A HREF="ftp://ftp.isi.edu/in-notes/rfc2683.txt">RFC2683</A> <DD> IMAP4 Implementation Recommendations <DD><A HREF="http://www.faqs.org/rfcs/rfc2595.html"></A> </DL> Using TLS with IMAP, POP3 and ACAP <HR> <table width="100%" cellpadding=0><tr> <td width="30%">Back to <a href="index.html">Fetchmail Home Page</a> <td width="30%" align=center>To <a href="/~esr/sitemap.html">Site Map</a> <td width="30%" align=right>$Date: 2000/06/07 00:22:00 $ </table> <P><ADDRESS>Eric S. Raymond <A HREF="mailto:esr@thyrsus.com">&lt;esr@snark.thyrsus.com&gt;</A></ADDRESS> </BODY> </HTML>