clc2112
In Mail under the Help menu, it will tell you how to delete Previous Recipients;
To manage addresses in your Previous Recipients list:
Choose Window > Previous Recipients.
If you want to sort the list, click a heading to sort by name, address, or the date the address was added to the list.
If you want to search for a name, enter it in the search field in the upper right corner of the window.
Select an address and do one of the following:
Click Remove From List to remove an infrequently used or misspelled address.
Click "Add to Address Book" to add a frequently used address. (Addresses that are already in your Address Book have a card icon next to them.)