I often receive calls saying that users emails addresses are wrong, or spelt incorrectly and they keep showing up in Outlook; it usually turns out to be just the auto prompt feature of Outlook remembering an old invalid address, here is a short guide on how to delete it:-
1) start typing the first few letters of the address you think is wrong
2) use the up and down keys on the keyboard to highlight the entry you want to delete
3) press the “delete” key on your keyboard…
4) DONE: you will not see that address in the list anymore!