How do I delete a previous recipient's email address from Mail?
Fire up a "New Message" from the File menu (or use command + N)
Begin typing the address
When Mail starts to complete the address you want, hit the Return key to tell Mail to go ahead and auto-complete the address
Hover the mouse pointer over the address; a blue arrow will show up on the right hand side.
Click on the arrow, which brings up a drop-down menu. The menu contains several options, one of which is "Remove address." Click it, and it will remove the address from auto-complete.