we develop the forum using PHP and SQL language. Thus, official functions that are provided by these languages are used. The mail function is one of the official function. We only have access to the so-called return value. In case of the mail function, the return value is true or false, which means:
"Returns TRUE if the mail was successfully accepted for delivery, FALSE otherwise.
It is important to note that just because the mail was accepted for delivery, it does NOT mean the mail will actually reach the intended destination."
Please note the second paragraph!
To validate your behaviour, I would create a simple (stand-alone) php script that sends an email using the parameter defined by the mlf-mail function. Having such a script, the mail function is independently of the forum software and, thus, can be validated.