But also, if you keep Contact Chips enabled -- once you've "autocompleted" an address, you can tap it (the "chip") and the app will 1) show the actual email and 2) let you pick among multiple emails of same contact.
Another tip:
Android Contacts has this concept of "primary" (email, phone) when there is more than one. AquaMail's autocompletion offers the "primary one" first.
The exact method for marking an email as "primary" can vary with Android version, device manufacturer, etc. -- but this would be in the phone's Contacts app, if you open a contact for editing.