i run an appVM with thunderbird only. I do have my own mail server and i also do have a google mail address in thunderbird.
When i enable the firewall in the email AppVM to only allow connections to known hosts and their email ports everything seems to work at first.
After a while though Thunderbird is unable to send out emails via google and only way to fix this is by going into the AppVM fw settings, clicking edit on one google mail setting and not changing anything. Just a press of OK and then apply in the AppVM settings window.
Voila i can now send again via google.
I do not know how or where i can see when this happens in logs to analyze further.
Used 993 for imap, and 465 587 995 then gave up and used any but problem still persisted.
I might be misunderstanding how this Firewall is supposed to work but something seems odd, it might also be a problem with google mail shenanigans.