I don't use domain.com but it looks like you might get to more granular DNS control via:
1. Log into your account
2. Click on the "My Services" Tab
3. Click on the "Manage Services" button, next to the domain using DNS Management
4. Under the "Other Services" heading, next to DNS Management, click the "Manage" link
.
If domain.com doesn't have some kind of full DNS management feature then you'll need to follow Chops' instructions by signing up with
http://www.dnsmadeeasy.com/ (Looks to cost about $15 a year)