I am having a problem sending emails from K-9 email client. I am using a BT smtp server and sending from my own domain email address via Android v11 and K-9 5.806. The incoming server is IMAP. The setup follows BT instructions including SSL and port 465. The email appears to go OK without any error message, but in fact is not sent and appears in neither Outbox nor (mostly) Sent folders. I have a similar problem with a tablet on Android 7.0 and K-9 5.600 except that in this case outgoing emails are saved to the Sent folder but not actually sent or received by the intended recipient. Tbird on my PC works OK with the same mailboxes. K-9 works OK with Utility Warehouse servers. This problems appears to affect the IMAP mailbox only: a POP3 mailbox is OK.
brendonxfx's problem (Android clients not sending emails - 01-02-2021) looked similar so I followed the solution instructions to change the account password, but to no avail.