My IP address is not dynamic, it is static. I could relay via the ISP but as a general principle that provides one more thing to go wrong, one more point of intercept etc.

And it provides another big risk. If a spammer joins your ISP and gets spam through that mail server, it can get blacklisted. Yes, sometimes your whole ISP’s network can be blacklisted by the worst of the crazed anti-spammers, but that’s less common.

Of course, they want you to route your mail through the ISP just so that they can blacklist more easily. They want to have somebody who will feel the pain if they are blacklisted.