Too late now, but I suspect the simplest thing to have done would have been to have the arduino hit a remote URL to trigger the email-delivery, rather than doing it entirely standalone.
Actually, now that you mention it I think that's what we did in the end: I wound up building a "private cloud" (really more of a pileus than a full cloud) just for this one thing, whereas the Pi let me put it all in one self-contained box that I haven't had to touch since.
