How to show actual email address in iOS Mail app?
If you see the word 'Details' at the top of the message (near the right side), tap it. 
Click on the persons name to open the contact card
Scroll to their emails
Look for the email with a "recent" tag above it.*


*In older iOS versions the correct email/phone number had a different color but in new versions it has a "recent" tag above the correct email/phone number.

See below: