This is an interesting issue. Bern, Ralf this ticket has a message that when process by 
the mail gateway resulted in a UunicodeDecodeError with python 2.7 and roundup 1.6.1
(run as issues.roundup-tracker.org).

I think this bug is fixed in the current development release as I don't get an error 
message when I run a current dev tracker under python 2 or 3.

Does this look like a reasonable way to test. I generated the test message using:

  (echo From roundup-admin@localhost.example.com; curl https://issues.roundup-
tracker.org/file1789/message_to_mailgw.txt ) | sed -e 's/\[issue[0-9]*]/[issue1]/' -e 
's/marcus@priesch.co.at/roundup-admin@localhost.example.com/' > unencode.msg

where roundup-admin@localhost.example.com is the address for my admin user.

This creates an mbox style mail spool file that I used as:

  python3 roundup/scripts/roundup_mailgw.py trackers/issue.tracker mailbox unencode.msg

I did the same with python2 and a python2 tracker homedir.

I verified that the characters that should cause the error were stored in db/msg/msgXX
by running 'od -c' on the file and verifying the 3 digit non-ascii character codes
on the line before the answer: header.

It might be worth looking at

  https://issues.roundup-tracker.org/msg7562

for background and

  https://issues.roundup-tracker.org/issue2550740 
  https://issues.roundup-tracker.org/issue2550540

for other still open encoding issues that sadly do not have reproduction steps.

Do you think I should try to set up a 1.6.1 install to test with to verify that
my test method can cause the issue?

If you guys think this is a valid test, I'll close this ticket out as not-reproducable.