One of our client was getting error when attempting to send email from Mail client:
[wpfmb type=’error’ theme=2]SMTP Error (-1): Connection to server failed[/wpfmb]
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
You just need to modify SMTP_ALLOWLOCAL to 1 to enable webmail to function.