Dynamic IP adresses can behave different. Some ISP change the adress every day, some other change only each 60 days or nomething like that (When your server is logged on all the time.)
I such a "slow dynamic" IP. I have written a small script that update the dynamic adress ip every hour. This have now worked without a problem for years.
I think it is not a big problem, but there exist a very few mailservers that refuse to send post to a dynamic ip. (Well known web mail like Hotmail and Yahoo is not a problem.)
To be on the safe side conserning this problem, I set up an acount with a fixed ip on a web hotel. Then I use the web hotel mail adress and my web hotel IP as contact info. Hven the mail reach the web hotell it is all redirected to the SME server with the dynamic IP.
So, today I can send the mail directely to my SME server or via the redirection via the web hotel.
Why then run a mail server when I already have a web hotel acount ??
Well, I use the Horde web mail on the SME, and I think it is a good idea to have a mail box of the size of some Gigabytes.