Chapter 20. The M (Mail Delivery Agent) Configuration Command

Other than relaying mail via SMTP or LMTP, the sendmail program does not perform the actual delivery of mail.[25] Instead, it calls other programs (called mail delivery agents [26]) to perform that service. Because the mechanics of delivery can vary widely from delivery agent to delivery agent, sendmail needs a great deal of information about each one. The sendmail M configuration command defines a mail delivery agent and provides the information that sendmail needs.

What’s New with V8.13

V8.13 sendmail has made four changes regarding delivery agents, three of which are described here and two of which are described in the specified sections:

  • The #error agent can now return a $@ quarantine with a reason (Section 11.1.2.6 [V8.13]).

  • Connections caching for the LPC delivery agent (Section 6.1 [V8.13]) is now provided.

  • The new F=B delivery agent flag strips all leading backslashes (Section 20.1.1 [V8.13])

  • The F=f flag has been removed from the defaults for local_lmtp (Section 20.1.2 [V8.13])

  • The new F=W delivery agent flag causes persistent host status to be ignored for an individual delivery agent (Section 20.1.3 [V8.13])

New F=B Delivery Agent Flag

The F=s (20.8.43[3ed]) delivery agent flag causes sendmail to dequote the recipient’s address before passing it to the selected delivery agent. Dequoting causes all quotation marks (“) and all leading backslashes () to be removed. But when a lighter touch is needed, you may use this new F=B flag instead, which just removes all leading backslashes. For example:

"\user"@relayhost    with F=s becomes    user@relayhost
"\user"@relayhost    with F=B becomes   "user"@relayhost

Note that because F=B is a subset of F=s, you are discouraged from using both flags at the same time.

No F=f for local_lmtp mc config

Prior to V8.13, the local_lmtp feature (4.8.19[3ed]) set the default LOCAL_MAILER_FLAGS (20.5.6.2[3ed]) to F=PSXfmnz9. Beginning with V8.13, the F=f flag (20.8.24[3ed]) is no longer set as part of that default. Recall that if sendmail is run with a -f command-line argument (15.7.23[3ed]), and if the F=f delivery agent flag is specified, the A= for this local delivery agent will have the two additional arguments -f and $g inserted between its argv[0] and argv[1].

The New F=W Flag

The HostStatusDirectory option (24.9.52[3ed]) defines where long-term host status should be maintained. If you prefer a particular delivery agent to ignore that status, you may do so by defining this F=W flag. When defined, it acts as though the HostStatusDirectory option were undefined for that particular delivery agent.



[25] For the purpose of this discussion, we gloss over the fact that sendmail actually can deliver directly to files (12.2.2[3ed]).

[26] These are misleadingly called “mailers” by the internal documentation supplied with sendmail.

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