Requisites for working multidrop mailboxes

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:

• Bcc: is removed from the header - it is supposed to be invisible to the recipients
• To: and Cc: are not updated to reflect the new target when mail is being redirected
• To: and Cc:, in mailing list mail, contain the LIST address and not the subscriber's address

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. 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. 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. 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).

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.