cross server php e-mail?

Thanks all of you for the e-mail scripts. But I cant use any of them because the server this is going to run on does not allow you to send out e-mails. So my question is...
Server "A" is hosting a web site that cannot send e-mial but supports php.
Server "B" is a ISP with a SMPT with a user name and password.

How do you get a php Form on server "A" to send a e-mail through server "B"?