Mails from posteo account not sent

I using two accounts in k-9: arcor and posteo. Sending mails from arcor works, sending from posteo doesn’t. I triple-checked the account:, SSL and the password. Receiving mails works on both. Is there a log that I can look at, to see what’s going wrong? Also tried posteo with fairmail-app and is working.

Would it be possible to make the debug log available without the need of external hardware / tools? The same is possible for other apps.

Sorry for the delayed answer, but I didn’t have access to a computer until now to use adb. I logged some tries to send via posteo, but couldn’t find any clues in the logs. So I just reinstalled k9, and it’s working now. stphns suggestion, to make the log available directly on the device, would be very useful.