Will somebody please advise. Where does Apple's Mail store its addresses?

I have used Mail for years, adding, removing and editing address without any problems. Suddenly one address refuses to be changed! No matter what I do, the original version always comes back. The new version is, of course, in the Address Book. 

I click New Message, type 's' and there is a list of suggestions which always includes the previous version of the address I need, never the new one.

I don't have much hair, or I would be tearing it!

Doug