Shoutbox

Hi there.
I have one person on my list (unfortunatly the only person I ever actually NEED to talk to) and neither of us can message eachother. Whenever we send a message we get a default message saying: "The following message could not be delivered to all recipients".
What can I do about this?

I use Messenger 7.0 and Messenger Plus! 3.40.

Thanks for your help.