[Sam Bauers]

PHP mail() only uses SMTP when PHP has been directed to use an MTA by default like in your case. There are plenty of situations where this isn’t the case, like on a lot of shared hosting services. In these cases the only way to send mail is to programmatically connect to an SMTP server. This is possible in 1.0-alpha versions using the plugin that chrishajer pointed out.

I wish it was as easy as changing that one line, believe me. Also, if your MTA is not accepting that very standard form of email address then it is really a config issue at the MTA. There is no reason it should be rejecting an RFC compliant email address like that one.