Outlook offers the function to remember previously typed email address. It stores these addresses in a Nickname cache (.nk2 file). As useful as this function can be, unfortunately Outlook also remembers addresses that you enter incorrectly.
When next typing the address, the erroneous address is offered as a choice. Without checking, it is easy to select that address and repeat the problem. Fortunately it is a simple procedure to remove the problem addresses.
When creating a new message, start typing the first few letters of the previously entered address.
Please note that the addresses have been blurred for the purpose of the screenshot.
The previously entered addresses appear. You can then use the cursor keys to navigate that list. Once the incorrect address is highlighted, press the Delete key on the keyboard. That address will be removed from the list.
NB. The NK2 file that controls what appears in the autocomplete list is associated with an Outlook profile. Should you change Outlook profiles, then a different/new NK2 file will be used.