Similar issue. K-9 Mail 6.603 installed via F-Droid on my new phone. Old phone used a very old version of K9 successfully. I am using a mail service called hostgator with IMAP. On PC I use Thunderbird with SSL/TLS (465), user@domain and password with zero issues. Outgoing emails from K9 do not arrive. They do not immediately appear in ‘Outbox’. I have gotten like 2 failed to send messages, but have sent a lot more test emails. I have been able to enter user@domain and password, selecting either SSL/TLS (465) or STARTTLS (587) and it seems to pass the password check, but no actual email is sent. One theory is that my service originally used user.domainname@providerspecificmachine for outgoing and K9 may have site specific code to fix the outgoing for this domain? Not sure what else it could be. If contacted privately I could set up an account for testing. My domain is psifoo.com, mail server is ‘mail.’ followed by the domain name. Thanks!