Is saslauthd running and did it start at boot time?
You perhaps could see if the file /etc/default/saslauthd contains a line with ‘START=yes’ (without quotes). In a situation like yours for me this line was missing after a clean U22.04 install and vm7-beta-install.sh version 7.0.0-RC8.
Only after I added this line I could send emails from Thunderbird @W10 and an android tablet. Before that I could receive mails and only send from UserMin.