When sending email messages using PHP, you must set the envelope sender in order to have bounces handled correctly. In order to set the envelope sender, use the -f sendmail option in the additional_parameters parameter of the mail command. Here is an example:
mail("firstname.lastname@example.org", "subject", "body of message", "From: email@example.com", "-firstname.lastname@example.org");
Without the last parameter, "-email@example.com", bounces would come back to a system administrator account, instead of the From address, which is usually not what is intended.