Recipient must be resolved - OUTBOX issue

For some reason this client tells me there is one email address in a message with an attachment I sent yesterday that must be resolved before it will allow me to poll new messages. When I click on Outbox I see no messages in there.

The Error reads:
“[Exchange Web Services] Sending email failed due to the following: At least one recipient is not valid…”

I know for fact the email is valid

I am on Exchange.

Using Outlook shows me nothing as far as accessing any Outbox.

I am not on any VPN

I cant use this program to retrieve any mail now until it is resolved.

Any Help ???

Thanks