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Suppose there is an instance of Sendmail 8.14.4 which handles incoming and outgoing email messages for domains a.com and b.com.

Now, what I'd like to achieve is the following: If Sendmail sends a message with a FROM of the form user@a.com, it should use mx.a.com as HELO name, and if it sends a message with a FROM of the form user@b.com, it should use mx.b.com as HELO name, i.e. I'd like to generate the HELO name for every outgoing message dynamically according to the envelope-from (SMTP MAIL FROM) or the From: Header, respectively.

Is this possible? Currently, I have configured the HELO name for all messages to be mx.a.com (using confDOMAIN_NAME or confHELO_NAME, respectively).

Binarus
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    What problem do you hope to solve with this? – Sven Jun 22 '16 at 15:40
  • I admit that it won't be a show stopper if this wouldn't be possible. On the other hand, I am curious, and, more important, I would find it much more logical and trustworthy (from a recipient's point of view) if the HELO domain would be the same as the FROM domain. – Binarus Jun 23 '16 at 10:03
  • That's what SPF records are for: They explicitly state "`mail.example.org` is allowed to send mail on behalf of `example.com`. – Sven Jun 23 '16 at 10:06
  • You are right, and indeed, I have implemented SPF and DKIM both for sending and receiving messages. But there are still a lot of people who do checking by other methods (for example, by comparing the HELO to the pretended sender's domain), and it still would be more logical if we could set the HELO per message. Some years ago, I have written one or two sendmail macros to achieve other things, and I am currently trying to remember that knowledge and to find out if I could get where I want by writing another macro (without success so far). – Binarus Jun 23 '16 at 11:51

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