Outlook is using the Auto-Complete (suggested recipients) list. Remove the wrong entry from that list so it stops filling in the misspelled address.
In New Outlook on Windows (Microsoft 365):
- Start a new email.
- In the To box, type the first few characters of the wrong address.
- When the incorrect suggestion appears in the drop-down list, move the mouse pointer over it so it is highlighted (do not click it).
- Select the X icon that appears next to the highlighted suggestion, or press the Delete key.
Next time, type the correct address in full; Outlook will then save the correct one to Auto-Complete.
If using Outlook on the web, the same idea applies: start a new message, type until the wrong address appears, highlight it in the suggestion list, and remove it.
For more details on managing these suggestions, see the guidance on fixing Auto-Complete entries and 5.1.x errors (the removal steps are the same).
References: