I can't figure out how to delete and email address for someone that is no longer valid.

Delete it from where ?

Echoing King_Penguin's query, "Delete it from where?"

I will take a guess that you need to change it > Manage your primary, rescue, alternate, and notification email addresses - rather than delete it

ÇÇÇ

Look in the Previous Recipients list, delete it from there.

Mail>window>previous recipients. Delete there.

Barry

Assuming that this is on your iOS device, then you can change a contact's details via the Contacts app. If it's not linked to one of your contacts then I get a blue 'i' to the right of the email address on the popup list :

Tapping on that gives me a second popup, at the bottom of which is :

Go into your email and click the icon to compose an email.

Tap on the area where you will add the address and begin typing it as you were. When it pops up the list of addresses, look to the right. There should be a blue "i" with a circle around it. Tap that.

At the top of the screen on the right corner, tap "Remove".

A menu will pop up at the bottom of the screen saying "Remove From Recents" or "Cancel". Select the former. That will get rid of it, and take you back to your draft email.

Now begin typing the address and select the correct address from the list, or select the blue circle with the "+" to the right. That will bring up your address list, where you can select your contact's email address.

My bad... I failed to read the title fully - ÇÇÇ