Click New Email from the outlook window.

Start typing an email to a frequent recipient that has an invalid email address. Hover your mouse over the right side of that address and click the delete button, or alternatively use your arrows keys to highlight the address and press the delete key on your keyboard.

Hover your mouse over the right side of that address and click the delete button, or alternatively use your arrows keys to highlight the address and press the delete key on your keyboard.

Repeat steps 2 and 3 to remove the remaining cached addresses.

Once you have removed the legacy addresses that were cached, find that person in the address list when you send them an email next time, and their new email will be cached for you.