Outlook will store email addresses that you have previously emailed in an "autocomplete" cache. Changes to that account can cause this stored address to stop working. This can be corrected by deleting the cached copy of the address so that it is recreated the next time it is used.
To delete a single autocomplete address:
1. Open Outlook and open a new message
Open Outlook and click on the New button to create a new email message.
2. Type in the email address
Type in the email address that you are having trouble sending emails to. This will cause the autocomplete suggestion to appear.
3. Select and delete the email address
If there is more than one suggestion select the email address to be removed by using the up and down arrow keys. Once the email address to be removed is highlighted, press the Delete key on your keyboard. This will remove the email from Outlook's cache and you should be able to send emails to that address now.
To delete the entire collection of autocomplete addresses:
From your inbox, go to File > Options > Mail > Empty Auto Complete List (under sending mail).