I've been working on a Drupal module to allow me to send mail with msmtp. I call it Mailer, which is a terribly creative name, I know. I'm so clever!
Mailer can be extended to work with any SMTP client, but for now has an interface for msmtp. I use msmtp for transmitting mail to the Google SMTP server.
Mailer works by opening a process with popen, and the writing the email into this handle.