If there are issues with sending to an existing email address, clearing the cached email address may resolve the issue.
To clear the cached address:
- Open a new email.
- Begin typing the email address and when Outlook auto-completes the address, click on the 'x' to delete the address.
- Retype the email address in full.
- Send the email.
Alternatively, to clear all auto-completing addresses:
- Select the "File" menu, and select "Options".
- Select "Mail".
- Scroll down to the "Send Messages" section, and click "Empty Auto-Complete List".
If you still have issues sending to the existing email address - in particular a known email address within your organisation - please refer to Hosted Exchange > Outlook > Update Offline Address Book.