Hi athegn,
So, Aqua Mail can suggest contacts from several different places.
1. E-mail recipients in your account
2. Phone contacts with an e-mail address
3. For accounts running over Microsoft Exchange, the exchange contacts that are stored on the server
You can see where suggestions come from by tapping the icon next to the To field when composing an e-mail. After that just search the e-mail you're interested in to find which category of contacts it comes from. Groups are also part of your phone contacts in that menu - they are contact groups.
If the e-mail address is suggested as part of #1, you can't remove just it, you have to disable suggestions entirely from Settings > Contacts > Recent addresses.
Martin