I've seen something similar happen when I answer to an email address that's not configured.
Let me explain better:
- In K9 I only have configured to receive and send from my main address. Let's say it's 'main@domain.com'
- I have several different addresses to my main address. Again, let's say it's 'secondary@domain.com.'
- If I receive one of those forwarded emails and reply to it, in the FROM you'll find 'secondary@domain.com'
- It will go to the outbox, but never get sent... but without any error. So you never know it wasn't get sent unless you check the outbox, and you won't know the reason.
I guess it's because there's no outgoing email profile for secondary@domain.com and it's lacking some sort of error management for that.
I can never get K9 to send email unless I manually sync right after. Some not very useful error pops up, then one sees the mail still in outbox, and the solution is a refresh.
The only thing that worked was to move the email from outbox to drafts (by editing it) and then sending again.
I wish this got fixed :-)