From 1ff7ec141e23c7d6f4eb292c035011b6158ad2bd Mon Sep 17 00:00:00 2001 From: Matthias Andree Date: Thu, 26 Nov 2020 11:14:47 +0100 Subject: Import multidrop documents, and link in FAQ. --- website/multidrop.html | 143 +++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 143 insertions(+) create mode 100644 website/multidrop.html (limited to 'website/multidrop.html') diff --git a/website/multidrop.html b/website/multidrop.html new file mode 100644 index 00000000..2de3efc3 --- /dev/null +++ b/website/multidrop.html @@ -0,0 +1,143 @@ + + + + + + + Requisites for working multidrop mailboxes + + + + + + + + Link zur deutschen Version/link to German-language version + +

Requisites for working multidrop mailboxes

+ +
+ Matthias Andree 2004-05-27 +
+ +

Introduction

+ +

Many ISPs offer a POP3 multidrop or "domain in a mailbox" + setup to their clients who can then fetch mail for multiple + recipients in their domain "in one go".

+ +

Often enough, such an undertaking goes awry during the mail + fetch, causes and remedies are to be presented in this + document.

+ +

Initial Examination

+ +

POP3, the Post Office Protocol version 3, was intended to + transport mail for a single recipient. It does not keep the + envelope that indicates the actual recipient and sender.

+ +

The envelope sender is often copied to the "Return-Path" + header, with respect to the envelope recipient, every + programmer of a mail server will have their own implementation. + Common solutions are "do nothing" (sendmail), "Delivered-To:" + (in qmail, potentially with a domain prefix; Postfix), + "X-Envelope-To:" (certain procmail-based setups) and + "X-Original-To:" (Postfix releases after 2002-10-25 will write + this in addition to Envelope-To:)

+ +

Important background information: Mail + headers such as To:, Cc:, Bcc: are IRRELEVANT for routing and + delivery of the mail. Mail routing will only look at the + ENVELOPE - there is no difference from snail mail here.

+ +

We will frequently see that upon injection, the envelope is + created from the headers, but ONLY THE ENVELOPE carries, in + contrast to the HEADER, the full information:

+ + + +

Dropping off mail for multiple distinct recipients in the + same mailbox requires the server to deposit the actual + recipient in the mail in order to achieve proper delivery. POP3 + makes no provisions, so this must take place outside the + protocol, the mail header lends itself to the task.

+ +

Requirements

+ +

POP3 multidrop can work reliably all the same, provided that + some requirements are met. These are:

+ +
    +
  1. The ISP MUST drop one copy per recipient in that + domain.
    2. + +
  2. The ISP MUST deposit the envelope recipient in the mail + header. Which one your ISP chooses can be asked from their + tech support or you'll see it when looking at a mail header. + You'll typically find X-Original-To:, X-Envelope-To:, + Delivered-To:.
    3. + +
  3. The POP3 client (Mercury/32, fetchmail, getmail, ...) + MUST reliably recognise the header where the envelope + recipient has been deposited and use ONLY this header for + mail delivery.
    4. + +
  4. The POP3 client MUST IN NO CASE evaluate To: or + Cc: headers. It MUST ON NO ACCOUNT feed the mail into a + command that is used for injection, such as + sendmail -t -oi (whereas sendmail with a fixed + local mail address, for instance, sendmail -oi joe, + is justifiable).
    5. +
+ +

Explanations

+ +
+
Ad 1:
+ +
If this requirement is not met, mails to multiple + recipients of the multidrop domain will only reach one of the + recipients.
+ +
Ad 2:
+ +
+ If this requirement is not met, delivery will be faulty. + Attempting to derive this information from the headers + (To:, Cc:) is dangerous and unreliable: + +
    +
  • Mail for mailing lists (which have their addresses in + the To: or Cc: header) will loop, which must be + avoided
    • + +
  • the regeneration of recipients that were placed in + the Bcc: header at the sender's site, is impossible + because the Bcc: header is removed for transport, as the + name "blind carbon copy" suggests.
    • +
+
+
+ + + + -- cgit v1.2.3