Found this on another forum may help
"What I found was that Outlook still had some Exchange adresses in ' it's memory' and the message to be sent included an exchange adress, whereas exchange was not being used anymore.
This could happen on systems which have used Exchange but have changed back to POP3 E-mail delivery.
Checking the to-be-sent messages is the key, remove the adresses that are exchange adresses and substitute them with the real POP E-mail adresses.
"
David Lerwill
"If at first you don't succeed go to the pub"