SMTP Error (-1): Connection to server failed

One of our client was getting error when attempting to send email from Mail client:

SMTP Error (-1): Connection to server failed

This error is generally happen due to a setting in the CSF firewall program or another firewall. It might be due to having the following set:

SMTP_BLOCK = 1
SMTP_ALLOWLOCAL = 0

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You just need to modify SMTP_ALLOWLOCAL to 1 to enable webmail to function.