A search regarding K-9 and the "no valid recipients" error returned only a few results - but none of them solved the issue for me. So after spending quite some time on this I thought it would be worth sharing a solution. First of all the error message was misleading as the recipient exists. Using another email client with the same setup I received SMTP Error 554.5.7.1.
It became obvious that the security settings must have changed on my email server and became more restrictive at some point. Before I was able to send using any.name[at]my-xyz.domain. Now it simply rejected any mail that I tried to send. There might be other ways but adding the actual sender address (that I use in the K-9 SMTP settings) to my email server (bound to the existing inbox there) solved the issue. It's not K-9 specific but the wrong error message is. If some K-9 developer reads this: you might consider to add the error code or indicate that this can be an authentication issue as well. -- You received this message because you are subscribed to the Google Groups "K-9 Mail" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
