Hey,
AquaMail integrates with the device’s Contacts/People application (which is in turn synced with your device’s @gmail.com account(s) and other accounts you may have configured, such as Google+).
The app will display a drop-down list of matching contacts as you enter them in any of the recipient entry fields (after two characters), matching on names and emails.
You can also press the contact picking button to the left of each recipient entry field to bring up the app’s own contact picker. The built-in contact picker filters contacts by email, has search, and a group tab where you can pick a whole group or individual group members.
It also has a “recents” tab which shows email addresses from recent messages, regardless of their existence in the device’s Contacts/People app, and an Exchange tab (for Exchange accounts) also with search.
It’s also possible to use the system contact picker using the respective entry in the Menu when composing a message (or making it permanent in the app’s settings under Contacts) Since version 1.6.0, the app offers Exchange contacts (from the selected account) in auto-completion (enabled by default, can be disabled by long pressing on the account on the app’s main screen -> Exchange options).
The app can also offer actual recent emails (not saved in Contacts) in auto-completion (disabled by default, can be enable in app settings -> Contacts).
Cheers,
Radoslav