I have K9 Mail v.14 installed on my Android v.15 phone. Always there are two errors:
Problem 1 - Synchronizing:
Most of the time when synchronizing mailboxes there is an “Authentication failure” on my Hotmail- and Gmail-boxes.
Problem 2 - Sending:
Also when I try to send a message with any of my accounts (own domain-mail, Hotmail, Gmail) K9 Mail gives me an “Error sending messages (retries exceeded)”. Most of the time I can send it by sending again. But it’s not done automatically: I always have to take action before it will be sent. And most of the time this will take hours, but mostly days or weeks to be able to send it.
Both failures has nothing to do with settings or configuration. Sometimes it works as expected, but most of the times I have to retry until it does. The configuration is ok because otherwise it wouldn’t work overall. I’m using the same (IMAP) settings per mailbox I use in Thunderbird on my laptop with which there are never problems.
So, before someone wants to let me try different settings: All settings are ok and functional (the same IMAP settings I use on my laptop with Thunderbird), but most of the time I need to try to resend it (several times) before it does.
Also: the Authentication failures on Hotmail and Gmail has nothing to do with wrong settings or a temporarily malfunction or weakness of the WiFi signal. With my phone authentication always goes wrong and with my laptop it is always ok. Even in my own home where the WiFi is optimal.
If your not even willing to entertain the possibility of network or app setting related problems, what type of suggestions are you expecting to receive?
I’m sorry. I understand the grumpyness. Maybe my message was not clear.
In my humble opinion I noted specifically the circumstances in which the problem occurs.
My network and WiFi are working fine with my laptop, my phone and my tablet.
It’s my fault forgetting to mention my Android tablet on which I have K9 Mail also installed. But of course, K9 Mail on my tablet is exactly the same way configured as on my phone.
So, Thunderbird on my laptop and both Android devices have K9 Mail installed as IMAP systems and completely the same configured. My network and WiFi are working fine, no problems there.
So these are the reasons for not seeing the point of changing a mostly working system. Therefore I wrote there’s no need to go through the process of checking settings and configurations: I did already a hundred times.
The only question I have is: why is ONLY my phone giving trouble sending mail?
And I hoped there would be someone with a creative idea of something not so obvious.
But a bit shocked by the irritated way of answering, I checked everything again, but now even the Android system. And now I discovered 1 (one) difference: the tablet has Android v.9 and my phone has Android v.15.
Don’t tell me… It couldn’t.. Could it?
Gues what?
While writing my send-problem with K9 on my phone, I came across messages about K9 Mail being incorporated in Thunderbird for Android. So I read about it and concluded I need to switch to the latter.
After sending my prvious reply I installed it on my phone and imported all settings with the QR-code (nice function!).
All my mail accounts and mail are in the new mail system and better: the problem seems to be resolved. Into thin air, because I didn’t make any changes, I just imported it all as the system offered.
Maybe it was as I read, K9 wasn’t supported since 2018! I didn’t know!
Anyway: for now the problem seems to be solved. If not, I will come back here…
Thanks for the effort of reading and answering my problem.
Hmmm, not sure where you got this info, but K-9 was last updated Nov 18, 2025 in the playstore. It and Thunderbird are basically the same - my guess is that your fresh install of K-9 “shook something loose”, some setting that got corrupted in your old K-9 install and now works properly again.
I’m glad it’s sorted for you, I just wanted to clear up the misconception of K-9 no longer being supported
