If you can send from your webmail successfully, the problem may be in the .nk2 file. In Outlook, when you try to send him a message, do you type the full email address out, or do you use the autocomplete? If you use the autocomplete, then try typing out the full email address. If that does not work, then delete the nk2 file and try again.

To delete the nk2 file, follow these steps:

For Outlook 2010

1. Within Outlook, click on the File menu.
2. Click on Options.
3. Click on Mail on the left.
4. Scroll down to the Send Messages section.
5. Click on the Empty Auto-Complete List button.
6. A box will appear asking if you're sure you want to clear the Auto-Complete List. Click Yes.