For my customers who have an on-premises Exchange Server, I usually set them up with some sort of store-and-forward service for their email. For example, Dyn.com's Email Gateway service: http://dyn.com/email/dyn-email-gateway/
This way, the customer points their MX records at the Dyn.com servers (mx.mailhop.org), and effectively functions as the "edge server". Dyn then relays it to the exchange server on a special port (e.g. 10025).
This is good for a lot of reasons - the small customers I deal with often have port 25 blocked by their ISP, or they are on a dynamic IP address and it is unsuitable for their server to be directly receiving mail from the Internet. Sometimes they don't want their office IP address exposed to the world, or they still want mail to be accepted even if their server is down or the Internet is out. It also provides them with a hosted antispam and antivirus solution.
Dyn.com's email gateway service is pretty rudimentary, and not very configurable, and the spam filtering isn't that great and these days I don't really like it anymore, even though it is cheap.
Are there any other competing services out there that I can use which do the same thing?