Hi Askelon
I'm no expert (Yet

), but heres my two cents:
For what I read in your post, you have yourdomain.xxx, which is hosted by a webhostingcompany. So what you wanna achive is that all http-trafic (port 80) will point at one IP (that of your webhostingcompany), and all your mailtrafic/SMTP_trafic (Port 25) will point at another IP (that of your home computer). Unfortunately that is not possible i'm afraid. yourdomain.xxx can only point at one place at a time. Some workarounds could be:
1) Configure your webhotel to forward all mails to your ip address. But if you want to get mails larger than your ISP allow, your in trouble.
2) Ask your ISP to forward all trafic on port 25 to your SME-server. Highly unlikely that they will do it though.
3) Let yourdomain.xxx point to your SME-server and then redirect the URL to your webhotel (which should be given another domain)
4) Host your domain yourself!
C Ya
Rune
Denmark