Sending an email from Roundcube results in: SMTP Error (-1): Connection to server failed.

Symptoms

Email cannot be sent from Roundcube and the following message is shown when attempting to send an email:

SMTP Error (-1): Connection to server failed.

An error similar to the following can also be found in the mail Exim log:

2022-11-28 22:55:43 TLS error on connection from [::1]:34936 (SSL_accept): timed out

Description

The tls_on_connect_ports Exim setting specifies a list of incoming SSMTP ports that should operate the obsolete SMTPS protocol and immediately set up a TLS session without waiting for the client to issue a STARTTLS command. When port 587 is added to the tls_on_connect_ports list, Roundcube is unable to connect to Exim.

Workaround

1. Log into WHM as the 'root' user.
2. Navigate to "Home / Service Configuration / Exim Configuration Manager."
3. Click the "Advanced Editor" tab.
4. Scroll down to the tls_on_connect_ports setting.
5. Set the value to 465, the cPanel default.
6. Scroll to the bottom of the page.
7. Click the "Save" button.

Additional resources

Exim Configuration Manager: Advanced Editor