This is open k9 issue: https github com thunderbird/thunderbird-android/issues/4602
(sorry, no direct link [yet?] - since I’m running into the 2 links for new users limit …)
I’ve shared there another workaround that works for me on current k9 versions: basically it’s editing as new the stuck message as new from the outbox and sending it from there.
Related reports:
- Emails stuck in outbox with poor 4G (2025-09-10)
- Messages stuck in outbox - #8 by belkka (2023-12-27)