Mail rules hell

I haven't done much research into this, but I'm open to suggestions. And please don't send a suggestion to get a PC - that is not going to happen.

Machine is a iBook G3 running OS 10.3.9 I use the Apple Mail program as my mail editor. In the first week of trying to email my boyfriend, I eventually realized that he wasn't getting my messages. I wasn't getting anything saying that they were rejected either, but they disappeared into cyberspace.

I think what was happening was that I was using Plain Text formatting in composing emails. Plain text is a popular way spam gets sent, so I think the MSN filters were grabbing it and discarding it. I switched to Rich Text, and that solved the problem.

The problem is that there's a listserv that I sometimes post to. It rejects my messages because they're in Rich Text. So I have to switch to Plain Text, and it goes through. Then switch back to Rich Text so that I don't get annoyed that boyfriend isn't replying to my emails when he doesn't get it.

Suggestions?