SMTP connection with Thunderbird times out Wednesday 19th November 2025 13:16:00

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.

Haraka by default asks for a client cert which we weren't really aware of before as we're not using it, clients just tell Haraka that they don't have one, and that's it.

When Thunderbird introduced a bug in client certificate handling, this turned into a real problem, though. It wasn't that obvious because only few Thunderbird users in general experienced that problem - because most mailservers don't request a client certificate.

The bug is fixed in current nightly and beta builds of Thunderbird, but as we don't work with client certificates anyway, it makes way more sense to disable even asking for one.

Posted 1 week ago by :username

Thunderbird ESR (140.7.0esr) doesn't have the issue, please downgrade while we'll submit a bug report to haraka.

Posted 2 weeks ago by :username

We updated our mailserver Haraka to 3.1 to (hopefully!) tackle the issue.

Posted 2 weeks ago by :username