Outlook Gremlins?

Hello. In the last 24 hours I have been unable to send any emails using Outlook to someone who's address has always been available by simply entering the first letter of their address...pretty routine stuff.

However, if I go to any other site & try to send this same person a Screenshot, Voila: the person's name is listed, and I can send an email!

So: how is it possible that Outlook has forgotten (?) the person, but Screenshot has not?

Thanks! Michael.