The following article will explain you how to remove the Unknown Address that show up when you are composing an E-Mail message in Outlook.
If you are seeing an unknown address showing on the To: contacts drop-down list, a pending Group Network invitation is causing this issue.
To remove it, follow either of the steps below.
Decline the pending Group Network invitation
1. Click Contacts.
2. Click View invitations.
3. Click on Groups.
4. Click No, thanks.
Change your Auto-complete suggestion setting
1. Go to Options, then More Options.
2. Under Customizing Hotmail, click Advanced privacy settings.
3. Under Auto-complete suggestions select Only suggest people in my contact list.
4. Click Save.