Set up Mailhandler with IMAP Gmail
Unfortunately, MAMP (even the Pro version) basically provides no support for G-mail.  For evidence, look at the comments on the official guide on the MAMP blog.

Although I haven't tried it, one of the comments suggests this solution.

And here's another way to set up postfix on Lion without relying on MAMP.

I too use MAMP, and instead of working with mailhandler I use the SMTP module instead, which allows Drupal to send messages locally independent of the local mail configuration.