i think you need a special library for using pop3. be aware that recieving mail is pop3 and sending it is sendmail
to send mail just use this function(*nix systems only i think
PHP Code:
mail("sendto@sentit.com", "subject", "the message\n this is sendmail", "from: [email]nowhere@nowhere.com[/email]")
notice the "from" field at the end, this is one of the additional headers, these can be used to describe differnt parts of the mailing process like the mailer, the mailer version nand all diferntt types of things. and it can be used to change to from field. this is why some hosts (and nearly all the free ones) dont have smtp or pop3 support with php. hope that helps