There is a message in your email box that uses a non-standard character-set, and your Outlook does not understand how to process it correctly. With the rash of botnet spam that has been circulating around for the last 2 months, these messages have substantially increased in frequency.
There are a few ways to get around it.
The first way (and the way I recommend), is to log into »mail.ct.metrocast.net
with your email address and password, and remove any messages you dont need (read: spam). 99 times out of 100, that will fix it. For the other time, it is one of the messages that is deemed "important", start deleting more messages, or make copies (eg: print them) before deleting them.
We can also empty your mailbox for you,if you are not expecting anything important, this is often the fastest and easiest way to rectify the problem.