"SMTP Error (-1): Connection to server failed" in Roundcube when Imunify360 is installed

Symptoms

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

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

An error similar to the following can also be found in the main Exim log at /var/log/exim_mainlog:

CONFIG_TEXT: 20XX-01-01 00:00:00 TLS error on connection from [::1]:34936 (SSL_accept): timed out

Cause

The Imunify360 software contains a firewall configuration in addition to its malware management capabilities. If SMTP Traffic Management is enabled in Imunify360, SMTP traffic can be limited, preventing the Roundcube services from communicating with the server locally.

Resolution

1. Log in to WHM as the root user.
2. Navigate to Imunify360 / Settings / General.
3. Locate the SMTP Traffic Management setting
4. Enable the option to Allow local.
5. Save the changes.

Additional resources

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

Exim Configuration Manager: Advanced Editor