Yes, i thought of that as well being a problem.

Suggestions

1) Text Ads
2) HTML Ads.
- you could make it so that you only sent it to html based email clients. This is going to cut more people off but it could be an option.
3) Set a limit on the amount of emails people receive?
4) Set a max size on a post, ie it is cut off after a certain amount with a link to it in the forum
5) Only alllow discussion type things in certain forums/categories not all of them.

In the end i think the email ads is not a bad idea. Looking around for advertising, it seems that email ads actually generate a higher CPM than web adds that people have an amazing ability to filter out. Text ads are so much easier to fit into contact they may work out better. (and of course less load on server, less bandwidth which all results in lower costs).

Rangersfan, as you are one of the people who could probably make the hack i am trying to pursuade you here !