Well this is interesting.

Bern, I spun up 1.6.1 under python2 on my test machine and was unable to produce
the error. I did need to add a date to the "From " mbox header line
(From roundup-admin@localhost.example.org Wed Jun 5 01:06:54 2022).
The message was properly added each time (although I did make the message
id unique otherwise the message was ignored).

So I logged into issues.roundup-tracker.org and running the same test (using my email
and a date stamp) did produce the error, however I also saw:

   sudo -u roundup /srv/roundup/env/bin/roundup-mailgw /srv/roundup/trackers/roundup 
mailbox unencode.msg
  /srv/roundup/env/local/lib/python2.7/site-packages/psycopg2/__init__.py:144:
     UserWarning: The psycopg2 wheel package will be renamed from release 2.8; in
     order to keep installing from binary please use "pip install psycopg2-binary"
     instead.
     For details see: <http://initd.org/psycopg/docs/install.html#binary-install-from-
pypi>.
  """)
  No handlers could be found for logger "detector"

the handler issue was unexpected. The rsswriter.py detector module to generate an rss
feed from the tracker was one that I copied from my sysadmin tracker. It opened
a logger for "detector" which wasn't defined. So I changed the code to open the 
"roundup" logger instead (and also changed an extension to do the same).

Now when running the same command I can successful submit the same message (after
changing the message id) multiple times without an error being emailed and the
message shows up in the web interface (with the non-ascii chars represented by a glyph).

So this looks like an exception or other error was generated in a reactor
and it triggered the error email but also aborted the finalization of the
message in the database somehow. I am not quite sure how an error in a
detector can do that.

Sadly trying that same thing on my test instances isn't producing the problem.
I have a different version of python2 so that may be an issue.