If you encounter such error while sending mails as :
“Horde error :Failed to set sender: firstname.lastname@example.org [SMTP: Failed to write to socket: not connected (code: -1, response: )]”
Check “SMTP_Block” option in CSF configuration file.
Make sure that “SMTP_Block” is set to “0”. So that it will not Block outgoing SMTP.
Restart CSF Firewall once the changes have been made.