I use MantisBT 1.2.6 on an Amazon Linux server. It has its own email-sending class layered on top of the PHP mail() function. Actually, it has other options to use smtp or sendmail directly, but I use PHP mail. Then PHP mail in turn uses sendmail.
I configured sendmail to use a smart host, which was working fine as I could write a little PHP program to send a message successfully through its mail() function.
But mail sent by MantisBT never arrived and did not bounce.
/var/log/maillog showed the MantiBT messages being sent to the smarthost successfully
(relay=my-smarthost-hostname) with "stat=Sent (ok nnnn qp nnnn)"
If I changed the /etc/mail/authinfo file to use the wrong password, maillog showed "stat=Service unavailable" for that relay.
So it seemed the problem had to be something subtle in the MantiBT code. I used xdebug to find the call it made to mail(), and get the values of its parameters, and extracted them to a separate php file to play with. It passed a number of headers in addition to "From:" in the fourth parameter, and I suspected that they might have been separated by just \n instead of \r\n as the RFC (2)821 requires, which caused the problem. But I added code to change \n to \r\n in the header string and that made no difference.
In the end, what caused the problem was the presence of a (correct) "Date:" header. When I removed that header from the 4th parameter to mail(), the mail got delivered immediately. So I edited the MantisBT source not to generate that header, and all was fine (I also edited the MantisBT source not to put a space between the "-f" option and the sender address).
So my question is just whether the presence of a "Date:" header in the 4th parameter to mail() is known to cause a problem sending through a smarthost. With a follow-on of wouldn't you expect either a bounce or a message in maillog if the smarthost didn't like seeing the Date header?
PS I actually called Network Solutions technical support about it when I saw the sent ok message in maillog, but before I isolated it to the Date header. Naturally that yielded nothing useful, just instructions for how to configure your email client for POP or IMAP :-)