Gmail keeps populating the To field with an out of date e-mail address that I no longer contact.
The email address isn't in my address book but I did use it a lot previously (the contact has since changed company and email address).
How do I tell Gmail to never show it to me again?
Delete their email address in Google Contacts.
Remember that anyone you reply to in GMail gets a contact record created for them!
To remove an unwanted autocomplete email address in GMail, remove the unwanted contact record.
Select "contacts" from the drop-down menu at the top left.
search for the unwanted contact in your address book by typing their name (or email, type it as you usually do in the to: field) in the search box
open the contact, then use the "more" menu at the top middle to select delete.
Once the contact is deleted, the autocomplete in the to: field for that contact name/email will no longer occur.
(sorry Blake Edwards, you were merely the example! No offense intended!)
Go to Contacts then search for the email address, select it, and delete it. It is probably under "All Contacts" and pops up because you used it frequently in the past.
As of July 2019, Google must have changed the setup, making outdated the other answers given.
What works right now is deleting the address in Google Contacts.
Simply searching your contacts may not be sufficient - but you can begin typing the email in the Search field and when it shows up, click on it. That will bring up a Contacts "card" for that email, even if it is not a full contact that you have created. Select the three vertical dots at the top right, and Delete is one of the options.
