|Sounds like the address is in your autocomplete list.|
The easiest way to get rid of it is to bring it up by starting
autocomplete (in your case type "yyy" into the TO field). Once the
list appears, highlight your autocomplete entry (with the arrow
keys) and simply hit the "delete" key. That will remove the
address from your autocomplete list.
Autocomplete lists in Outlook are stored in a file that ends in .nk2.
If you want to remove all of your autocompletes, you could find and
delete that file (just google it) but you would lose all of your
autocompletes, including items in your address book.