Aqua Mail has the ability to integrate tightly with your system contacts by displaying your contact names and photos. There are also options to indicate unknown senders (not in your contact list) and auto-save your message recipients in your phone's contact list.
These settings can be modified via the Contacts category in the app.
Note: Contacts permission is required for this and the app will request access.

Another option ease of use option is "Recent addresses". The app will provide you with a suggestions list of e-mail addresses you've received or sent mail to recently. These do not have to be from your phone's contacts list, though if the permission is granted it will be used as well.