mail() source code

When mail() sends, via sendmail, an e-mail to an invalid address, it's returned to the "envelope-sender" (e.g.: the ISP), NOT returned to the "from" address, or the "return-path" address, etc. that are included with mail()'s 4th parameter. I'm planning to write a "modified" mail() to include the "true" return-path. In order to do so, I need to communicate directly with sendmail and, in my newbie's opinion, a good start would be to review the source code of the mail(). Where can I find it? Thank you! Anthony F. Rodriguez (afrodriguez@att.net)