🚀 go-pugleaf

The 4.0 sendmail.subsidiary.cf as distributed from Sun will cause mail
looping in some cases.  For example, suppose you are stk@topgun.frame.com.

Then sending a message to this address in the format above (or as
frame!topgun!stk) will loop between frame and topgun. This affects any
message to a 4.0 machine which includes the hostname (e.g., return mail
back to you, or mail from the outside where they include your hostname).

The fix: 
In your /etc/sendmail.cf file, search for # DmPodunk.EDU. 
Add 2 lines after it:

Dmframe.com
Cmframe frame.com frame.uucp

That's all there is to it. 

Notes:
1) You should replace "frame" in the example above with your mailhost's 
   domainname.
2) If you already have a Dm line, you don't need to change it.
3) The domainname appears 3 times on the Cm line.
4) To make the change effective, kill sendmail and restart it.

Here are the technical details:
=============================================
First, the Dm line really has nothing to do with the mail looping problem,
but it makes your mail headers look nicer.

The Cm line fixes the looping problem. The rule provided by Sun never
worked because $= works on classes, not macros. Since there was never a Cm
anywhere, the rule for S6 never matched the intended case.

As a sidenote, $= is supposed to match only single tokens. Apparently, it
is more clever than that, so this code works.