Automatic Send alert using Mail() function if SMTP fail
As most of users uses the SMTP for their out going mail in whmcs for alert like New Tickets, New Order or etc.
But what happen if SMTP fail to send mail. If this happen then you will not receive the mail. And this will leads to miss the important mail.
So this situation may be solve this way, when smtp fail to send the mail then php try to send mail using mail() function, and also send an alert about smtp fail to admin user(s).