SMTP Error (-1): Connection to server failed

When trying to send emails from Roundcube,Horde and SquirrelMail if you are receving the error as follows :

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SMTP Error (-1): Connection to server failed
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This error is typically seen due to a setting in the CSF firewall or another firewall. It might be caused by having the following set :

SMTP_BLOCK = 1
SMTP_ALLOWLOCAL = 0

You would need to change SMTP_ALLOWLOCAL to 1 to enable webmail to function.