You are getting bounce mails like this:
----- Transcript of session follows ----- ... while talking to mail.example.com.: >>> MAIL From:<email@example.com> SIZE=334 <<< 501 5.1.8 <firstname.lastname@example.org>... Domain of sender address email@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
firstname.lastname@example.org). 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
To solve this problem, get an "official" address for your gnats system, e.g., email@example.com, and use that one for GNATS_ADDR.