I recently downloaded the app and configured three accounts.
Auto configuration was used. But mails are not going from any account. Sent folder shows the mail, but it never reaches the destination.
Tried changing the outgoing settings , but none worked.
How to correct the issue
When the message has been uploaded to the Sent folder, your provider accepted the message for delivery. Contact your email provider to ask if they were successful in delivering the messages to the destination. If that’s the case, try asking the destination’s email server operator why your messages don’t arrive at the recipient. It might be classified as spam.
There’s been an issue in the past where providers “accept” mails sent by K-9 and then silently dumping them.
It correlates to the IP address used in the header fields. For privacy reasons, 127.0.0.1 (localhost) can be used there in order to not expose the actual IP. And exactly this is what some providers don’t like (or they have misconfigured their MTA without malicious intent).
But what is the solution. I tried sending mails to id hosted on same server
As long as @cketti insists on adhering to standards and that everybody else is doing it wrong (technically he is right, but where is the point if you are the only one swimming against the stream), the only option you have is to ask your provider to change their configuration and accept such emails. Or at least to not silently drop them and send you a reject message instead.