Please go to Configuration > General Settings > Mail tab and verify that the account details are correct and re save your details then try it again.
If it fails try the following:
1. Is the mail server you are wishing to connect to on the same server as WHMCS?
If it is, try connecting to localhost
2. If you wish to connect to an external mail server - you should ensure that the port is open on your servers firewall to allow outgoing connections on port 25,
Try mail@domainname.com or just domainname.com or just the IP of the mail server
If above fails try the following:
Please place the following line in your configuration.php file and try sending mail again it should output some information to view for any errors while trying to send mail via smtp. Copy and Paste here any message shown. Thanks
$smtp_debug = "on";
Once you have the error message please remove the
$smtp_debug = "on";
From the configuration.php file