Mails do not get sent to users when exim is used as mail daemon (sendmail with -t command).

Migrated from GitHub issue #4866.

GitLab improperly uses -t with sendmail, which seems to have inconsistent behavior between implementations. Using -t with exim causes message delivery failure with the following message:

A message that you sent using the -t command line option contained no
addresses that were not also on the command line, and were therefore
suppressed. This left no recipient addresses, and so no delivery could
be attempted.

Exim's -t documentation

-t When Exim is receiving a locally-generated, non-SMTP message on its standard input, the -t option causes the recipients of the message to be obtained from the To:, Cc:, and Bcc: header lines in the message instead of from the command arguments. The addresses are extracted before any rewriting takes place and the Bcc: header line, if present, is then removed.

         If the command has any arguments, they specify  addresses  to
         which  the message is not to be delivered. That is, the argu-
         ment addresses are removed from the recipients list  obtained
         from  the  headers. 

The most recent suggestion from the GitHub thread is that the bug lies in upstream rails, and that this issue should be reopened: https://github.com/rails/rails/issues/1755

Not an expert here, just encountered the issue, found it on GitHub, and noticed that it needed to be migrated here.