I think this is more of a DNS issue than mail server. You will need to add another A record to your DNS called mail.domain.com that points to your servers IP address just like you did for the
http://www.domain.com A record.
The webmail service is pre set to monitor port 8000 so
HTTP://mail.domain.com:8000 should pull up the webmail login screen. If it does not, check the template assignments in the ability mail settings.
I do not know that much about Apache but on IIS I had to move its smtp service to a port other than 25 so it would not compete with Ability.
If you do not have a static IP you will lose connection to both your mail and web server if your IP changes.
http://www.no-ip.com/ is one company that offers a solution for those running servers on a dynamic IP.
Good Luck