I thought of this as soon as I saw your post:
You can go to noip.com and purchase Mail Reflector (for incoming) and Alternate Port SMTP (for outgoing), and set up SM to work with this. The cost seems to be about $ 60/year for both. They will "reflect" incoming mail to an alternate port, and you can send through their "smarthost" on a different port than 25 (you relay your mail through noip.com). AFAIK, pretty easy to set this up in SM.
It's pretty reasonable IMO. I haven't used this particular service, but they've been doing it for a few years, so I'll bet they've got it down.
Note that I couldn't find Alternate Port SMTP on their Web site, but it was available as an option after selecting Mail Reflector.
Please double-check with them before jumping into this. For instance, is your public IP dynamic or static, and/or do you have a domain with DNS pointing to your public IP? Noip also offers dynamic IP services (these I HAVE used). As I said, I haven't used the email services, but I'm familiar with other noip.com offerings.
Hope this is helpful!