SMTP Error (-1): Connection to server failed

Symptoms

When attempting to send an email from a webmail client such as Roundcube, you are presented with the following error:

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

Description 

This error mentioned above is typically caused by having the following setting enalbed within the CSF firewall :

SMTP_ALLOWLOCAL = 0

Workaround

Please change this setting to the following instead:

SMTP_ALLOWLOCAL = 1