Do you use TrackerControl from Google Play/F-Droid/other?
F-Droid
Describe the bug
TrackerControl by default blocks internet traffic to some email providers, which causes Thunderbird for Android (previously known as K9-Mail) to run into email retrieval issues. It is unable to retrieve any emails from the servers.
The following domains are blocked by default:
imap.gmail.com (GMail)
login.microsoftonline.com (Outlook)
login.live.com (Outlook)
I have domain-based blocking enabled, not sure if this is related.