You can turn on synchronization of contacts from your mailbox and phone address books. This will enable you to select recipients (and see suggestions) from the address book of the mailbox and the device memory when sending emails using the app.
To enable synchronization, tap “Menu”, then “Settings” and check the “Address Book” box.
On devices under Android Marshmallow and later versions, the app will require permission to access device contacts. If you denied these permissions to the app, synchronization will not be possible. You can change the permissions any time using the device settings.
You can also copy individual contacts from your mailbox to the phone memory. To do this, open a message from or to a contact you want to save on your phone, then tap on his or her name in the “From:” or “To:” field (to see the “To:” field, tap “Details”). On the next screen, tap “Save on Device”. You can save the address as a new contact on your phone or update an existing entry.