If your app is really getting this involved, I'd recommend that you
just set up a real mail server on your Rails host and do it properly.
Also note that the Gmail for Domains stuff has (at least in the free
version) a hard limit of 500 unique To: addresses per day.
Alternatively, you could also look into a service like AuthSMTP, and
just set up a postfix instance that forwards the mails to them.