Well there's quite a few things lacking if you ask me, CC, BCC, attachments, batch mode on/off. I don't know if these are mail() constraints, I never use that thing. Basic things in email protocols are left out I believe because you could hack things up with the headers method, but that's far from an obvious thing especially for beginners. I don't have much preference on adding to the WireMail class, or having a supplemental interface to implement, or else.
Personally I believe the interface specification for send should not be an int, but a true-ish or a false-ish value. Many ways of sending mail don't return numbers sent, but will tell you if it went ok or not. By the very nature of email, it could take hours/days until an email is certified as being accepted by the receiving servers, and some email servers don't even return accept/decline messages.
true/1…n should be correct return values for success, false/0 should also be correct for failure. It's just a matter of documenting it and using == instead of ===. It's all fun insisting on integers but if we can't realistically meet those requirements it's just for show..