As noted by Paul Burd, .Mac e-mail aliases do not function properly on the iPhone.
"If you use aliases with your .Mac account, and you have an iPhone, be careful who you reply to. An email sent to your .Mac alias can be received just fine on your iPhone, but if you reply from your iPhone, your REAL .Mac address will be used. The iPhone canâ??t send from your alias, like your desktop app can."