Follow these instructions which I copied here for you:
Q&A: Removing Old Addresses from the Mac Mail Memory
By J.D. BIERSDORFER
Q.
When I use the Mail program on my Mac, it automatically completes the e-mail address I enter — even if it’s an incorrect or outdated address. I am sorry to say I have far too many sent e-mails to wrong or outdated addresses as a result. How can I eliminate them from the computer’s memory?
A.
Apple’s Mail program stashes away a list of previously used addresses to help speed up your e-mail chores, but it saves these addresses whether they are correct or not. Fortunately, it’s possible to open up the list and weed out all the unwanted, outdated or incorrect addresses at one sitting.
To do so, open the Mail program, go to the Window menu and choose Previous Recipients. The Previous Recipients window opens with a sortable list of the names and e-mail addresses of people you have corresponded with in the past, as well as the date of when the address was last used. Click the Name, E-mail or Last Used bar at the top of the column to sort the list on your preferred criteria.
To remove someone from the Previous Recipients list so that Mail won’t try to auto-complete the address, click on the entry to select it and then click the Remove From List button in the bottom left corner of the window. You can select multiple entries at once by holding down the Mac’s Command key to highlight several addresses as you go. Click the Remove From List button to zap all the selected addresses at once.
If you find a contact that you’d like to officially add to the Mac’s Address Book program, select it and click the Add to Address Book button in the bottom right corner of the window.
Let us know if this works for you.