Do you use the autocomplete feature in Gmail to remember email addresses? If you do, and you correspond with people that have changed their email address I have found a way to force Gmail to forget the old addresses!
Click on Google apps hamburger (the 9 dots in a square in the upper right corner)
Click on Contacts
Click on "Other Contacts" in the left hand column
Search for the old email address
Click on the address
Click on the three dots in the upper right for More Actions
Click on Delete
Click on Delete to confirm