[exim] Help with Mailman3

I just installed Mailman3 on my Ubuntu 20.04 LTS box. I could swear I did
everything correctly, but when I run exim -bt to test for a list request
address it says no such user.

Here's the output. Any ideas are helpful!

root@jupiter:~# mailman create testlist@???
/usr/lib/python3/dist-packages/pymysql/cursors.py:170: Warning: (3719,
"'utf8' is currently an alias for the character set UTF8MB3, but will be an
alias for UTF8MB4 in a future release. Please consider using UTF8MB4 in
order to be unambiguous.")
result = self._query(query)
/usr/lib/python3/dist-packages/pymysql/cursors.py:170: Warning: (1300,
"Invalid utf8 character string: '80055D'")
result = self._query(query)
Created mailing list: testlist@???
root@jupiter:~# exim -bt testlist@???
testlist@??? is undeliverable: No such person at this address