I get the following message from windows Live Mail:

The message could not be sent.

Some of the recipients for this message are not valid

Please verify the names.

This happens on 2 different email addresses to the same person (I can receive messages from these addresses, but I can't reply or send them. Previously, I didn't have a problem with these email addresses), and I can send them emails using my wife's computer (also using Windows Live Mail).

Can you help, this is very frustrating.

*** Email address is removed for privacy ***