sendmail, 4th Edition

H_ACHECK Header Flag (V5 and Later)

The H_ACHECK flag marks a header that should normally be discarded unless a delivery agent’s F= flag calls for its inclusion. It is usually set for the Bcc: header, which is discarded for the privacy of a blind carbon copy list, and the Full-Name: header, which is intended as a way for a user to add a full name (see the $x macro, $x on page 851) when there is no full name defined in the passwd(5) file. Note that H_ACHECK, when combined with bogus ?flags? of a header configuration file declaration, can cause appropriate headers to always be deleted or replaced. Also note that under V8 sendmail, the H_ACHECK flag alone always causes a header to be replaced.

Replace headers with H_ACHECK

Some MUAs tend to insert their own Message-ID: header (Message-ID: on page 1159). This can cause difficulty when tracing email problems because those MUA headers lack the sendmail queue identifier. At sites that have a central mail hub machine, where client machines forward all mail to the hub, you can solve this problem by redefining Message-ID: in conf.c on the clients, to delete the bogus Message-ID:, so that a good one can be generated on the hub:

"message-id",           0,
"message-id",           H_ACHECK,       ← change to this

Here, we changed the 0 flag for the Message-ID: header into an H_ACHECK flag. We do this only on the client machine versions of sendmail but not on the hub. The Message-ID: header will then be stripped from every outgoing message on every client machine and a new ...