SMTP connect() failed Error with External SMTP Mail Providers
Symptom
When sending emails using an external SMTP mail provider (including Gmail™), a SMTP connect() failed error displays in WHMCS and appears in the Configuration > System Log:
SMTP connect() failed. https://github.com/PHPMailer/PHPMailer/wiki/Troubleshooting
If you have enabled SMTP Debug for your mail provider in the Mail tab at Configuration > System Settings > General Settings, the log entry will also include the following details:
SMTP Debug: Connection: opening to ssl://smtp.gmail.com:465, timeout=300, options=array()
SMTP Debug: Connection failed. Error #2: stream_socket_client(): SSL operation failed with code 1. OpenSSL Error messages: error:1416F086:SSL routines:tls_process_server_certificate:certificate verify failed [/path/to/whmcs/vendor/phpmailer/phpmailer/src/SMTP.php line 375]
SMTP Debug: Connection failed. Error #2: stream_socket_client(): Failed to enable crypto [/path/to/whmcs/vendor/phpmailer/phpmailer/src/SMTP.php line 375]
SMTP Debug: Connection failed. Error #2: stream_socket_client(): unable to connect to ssl://smtp.gmail.com:465 (Unknown error) [/path/to/whmcs/vendor/phpmailer/phpmailer/src/SMTP.php line 375]
SMTP Debug: SMTP ERROR: Failed to connect to server: (0)
SMTP Debug: SMTP connect() failed. https://github.com/PHPMailer/PHPMailer/wiki/TroubleshootingFor more information about enabling SMTP Debug, see Email Sending Issues.
Cause
The server that hosts your WHMCS installation is blocking connections to the external SMTP server.
Solution
Adjust your mail server configuration to allow connections to external SMTP servers.
For steps to resolve this issue on the cPanel and Plesk control panels, see:
For help making this change, contact your hosting provider or system administrator.