Thunderbird (often) fails to send mail via SMTP - in the client this looks like a timeout, which isn't the case.
Thunderbird log:
mailnews.smtp: Command failed: 421 Unrecognized command;
currentAction=_actionEHLO SmtpClient.sys.mjs:578:19
09:17:18.868
mailnews.send: Sending failed; An error occurred while sending mail:
Outgoing server (SMTP) error. The server responded: Unrecognized
command., exitCode=2147500037,
We suspect a (new) bug in Thunderbird. In our logs, we find the client-side “STARTTLS” on a connection that is already running over TLS, which in this case is in fact an unknown server command sent by the client. Thunderbird fails to send mail via SMTP — connection timeout despite server reachable.
We are aware that there are mail providers that are unaffected by this bug, in contrast to many others we use Haraka and suspect the bug somewhere in the newly revamped certificate handling in Thunderbird, maybe only with Haraka.