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Binarus
Asked: 2018-09-11 01:30:14 +0800 CST

How to make Sendmail add an evelope recipient(s) header to all inbound email messages?

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Disclaimer: I have read some of the questions and articles here and elsewhere which deal with Sendmail rewriting headers. I haven't found an answer to the question below because the other question mostly relate to rewriting headers (instead of adding them), and nearly all of them relate to outbound messages and the envelope sender (instead of the envelope recipient).

Having said this:

I am running a mail server, using Sendmail 8.14.4 under Debian jessie.

There are some email recipient addresses which are mapped to the same O/S user account. When the respective O/S user reads the messages, he can't determine to what email address the messages originally have been sent.

Therefore, I would like to have Sendmail add a custom header containing the envelope recipient(s) to all inbound email messages.

I think I could better explain my problem by example:

Suppose I have two email addresses, [email protected] and [email protected]. Both are mapped to the O/S user account offers. This works so far: The O/S user offers, when fetching his email, gets all messages sent to [email protected] and [email protected].

The problem now is that he can't determine to which email address each message originally has been sent. That means: Although he is seeing all messages which have been sent to both email addresses, he can't tell if a certain message has been originally sent to [email protected] or to [email protected].

Therefore, I would like to add a custom header to each inbound message, perhaps something like that: X-Envelope-Recipient: <Original envelope recipient>.

What would be the easiest way to do this?

Once upon a time, I have written some simple custom rules for Sendmail. But nearly 15 years have passed since then, so I'd like to avoid that, and thus I hope that there is an easy solution or that somebody could point me in the right direction. To be honest, writing a milter to solve my problem currently seems easier to me than re-learning Sendmail's rule syntax ...

EDIT 1

As requested by @AnFi, here is the local mailer definition from sendmail.cf:

Mlocal,         P=/usr/lib/sm.bin/mail.local, F=lsDFMAw5:/|@qPSXnz9, S=EnvFromSMTP/HdrFromL, R=EnvToL/HdrToL,
                T=DNS/RFC822/SMTP,
                A=mail.local -l -h inbox
sendmail

2 Answers

  1. What you suggest would be against the SMTP protocol: there's legitimate reasons for adding addresses to RCPT TO command despite they do not exist in the To: or Cc: headers, i.e. RFC 5321 7.2. "Blind" Copies (emphasis is mine):

    Addresses that do not appear in the message header section may appear in the RCPT commands to an SMTP server for a number of reasons. The two most common involve the use of a mailing address as a "list exploder" (a single address that resolves into multiple addresses) and the appearance of "blind copies". Especially when more than one RCPT command is present, and in order to avoid defeating some of the purpose of these mechanisms, SMTP clients and servers SHOULD NOT copy the full set of RCPT command arguments into the header section, either as part of trace header fields or as informational or private-extension header fields. Since this rule is often violated in practice, and cannot be enforced, sending SMTP systems that are aware of "bcc" use MAY find it helpful to send each blind copy as a separate message transaction containing only a single RCPT command.

    There is no inherent relationship between either "reverse" (from MAIL, SAML, etc., commands) or "forward" (RCPT) addresses in the SMTP transaction ("envelope") and the addresses in the header section. Receiving systems SHOULD NOT attempt to deduce such relationships and use them to alter the header section of the message for delivery. The popular Apparently-to header field is a violation of this principle as well as a common source of unintended information disclosure and SHOULD NOT be used.

    The non-recommended Apparently-to header is controlled with option NoRecipientAction=action.

    Set the behaviour when there are no recipient headers (To:, Cc: or Bcc:) in the message to action:

    • none leaves the message unchanged,
    • add-to adds a To: header with the envelope recipients,
    • add-apparently-to adds an Apparently-To: header with the envelope recipients,
    • add-bcc adds an empty Bcc: header, and
    • add-to-undisclosed adds a header reading 'To: undisclosed-recipients:;'.

    Please note that normally the original recipient address is already in the To: or Cc: header. It should not be modified to the user account name offers, so it's either [email protected], [email protected] or hidden. Rewriting headers has become even more dangerous as it might also broke DKIM signatures. The only reasonable use case is address rewriting for mail with a local origin (user to [email protected]).

    Adding a list of all RCPT TO addresses would violate the protocol, but you actually simply need the single original RCPT TO address for the user the mail was delivered to. I don't know how to achieve this with Sendmail, but Postfix (with the default configuration) adds an X-Original-To: header containing exactly that, and additionally a Delivered-To: header containing the internal destination mailbox ([email protected]).

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  2. Best Answer

    Your question is addresses by Sendmail.org FAQ 3.29

    3.29 How can I add a header specifying the actual recipient when having multiple users in a virtual domain go to a single mailbox?

    Short version: Use virtusertable and ~offers/.procmailrc

    virtusertable:

    [email protected]  offers+offer1
[email protected]  offers+offer2

    ~offers/.procmailrc should get "plus detail" in $1.
    OR
    You can use $h (set to +detail) in custom headers

    Warning: you will get/process two copies of a message addressed to both [email protected] and [email protected]

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