This is a path that starts with you calling telnet from php, and ends with you reinventing sendmail. Trust me: just send the damn mail. The world of SMTP is a

This is a path that starts with you calling telnet from php, and ends with you reinventing sendmail. Trust me: just send the damn mail. The world of SMTP is a dynamic, volatile place and you should let your transport agent take care of these things. Otherwise you're going to be bumping up against MX relays (which will answer "yes" to any user at that domain), spam filters that will take any email, sender verification failures, SPF, and a whole lot of other things that will distract you from running your site.