Please do not submit or serve any configurations without SSL! There's no reason in 2010 why users still need to read mail entirely unprotected.
If you are an ISP and think the server load is too high, try adding an SSL accelerator. They are cheap and widely used. In fact, even most freemail (!) providers these days support SSL, so if users pay you money for ISP service, that's all the more reason to give them proper service. But first simply try to enable software SSL - small servers may be fine with SSL and without any additional installations.