I’ve had exactly the same problem since Monday 24th January. K9 will not work on ionos pop3 accounts.
But K9 works with Ionos imap configurations for the same email accounts, so does that mean it is something Ionos has changed, not a problem with K9?
I have the same problem since today with 3 android devices. No mail receipt. I installed another app as a test and configured an existing mail account with POP3 and it works. All mail clients on Windows devices (Thunderbird/Outlook 2021) work perfectly with this POP3 account. (IONOS/1&1) in Germany !! So it’s pretty sure it’s K9!
Supplement: Without changes, an account of 3 now goes with the reception. If you go into the configuration settings of all three accounts and check the reception settings, all three still show a POP3 error “Unable to execute POP3 command”. Furthermore, after deleting the mail on the Windows desktop, it will continue to be displayed in the app. The settings should also delete this after a new sync!
This was caused by a change in server configurations that gmx and ionos made. See the 5.910 beta version released yesterday (url above) or wait a day or two for that to be released as the new production version.
Supplement 2: K9 Beta works
Auf meinem Galaxy Tab S7+5G funktioniert die Beta-Version von heute. Auf meinem Galaxy S10+ (beide Android 12) bekomme ich weiterhin den POP3 Server-Fehler.
Confirm (in settings/about) that you are using the updated beta 5.910 on your Galaxy S10+. [It should reload automatically after the application is updated.]
If you are, then please provide details on your mail service provider, ports, security settings, etc. – see my list in the second post, above.
There is no beta Version in Google App store available for me.
Besides, Why is there no error message anyway? I only found out about the issue because people complained about missing answers. Otherwise I would not have noticed the missing Mails at all. Other mailing Apps inform users about errors… Could you please add this Feature to this Great App?
See the announcement of the new production release, which addresses the gmx/ionos pop3 issue, in forum item 4056.