Mail from gnats bounces

You are getting bounce mails like this:

        ----- Transcript of session follows -----
     ... while talking to mail.example.com.:
     >>> MAIL From:<gnats@mymachine.subdomain.example.com> SIZE=334
     <<< 501 5.1.8 <gnats@mymachine.subdomain.example.com>...
           Domain of sender address gnats@mymachine.subdomain.example.com does not exist

Probably your gnats sits behind a firewall, but it needs to exchange mails with the world outside the firewall. Make sure that GNATS_ADDR is valid outside.

This is what probably happens behind the scenes: On its way to the outside world, the sender address of the mail gets rewritten to something "official" (like Hans-Albert.Schneider@example.com, as opposed to me@mymachine.subdomain.example.com). This is especially necessary if the internal hosts are not visible outside. If your outgoing mail gateway still sees the internal address, it complains with a message like the above (it has probably checked mymachine.subdomain.example.com with your organization's external DNS server, which does not know mymachine).

To solve this problem, get an "official" address for your gnats system, e.g., tool-bugs@example.com, and use that one for GNATS_ADDR.