I'm generally sick of managing my own e-mail server, but there are some applications I'm working on that will be needing to communicate with 3rd party users over e-mail. For things such as registration confirmation, alerts, and account recovery.
I've been impressed with Gmail for personal use, and trust in Google's ability to handle arbitrary volume of requests, but is this scenario suggested?
The only requirements that I know of aside from using a business account, is that the applications will need to talk to their SMTP directly and over TLS/SSL.
If you have decent alternative mail hosting providers you like that are as reliable and reasonably priced, feel free to suggest them.