A sales agent in my office tried to send an email and got this error. I googled the error, and there were no hits! (That's never happened to me before - I'm usually not that original!) Does anyone know what this error means or where I should start digging?

Could not deliver message to the following recipient(s):

Failed Recipient: [email protected]
Reason: Remote host said: xxx x.x.x Your server IP address is in the UCE
layer 1 database, bye

(The x's were more realistic values in the real error, not sure if I'd be revealing anything I shouldn't if I didn't x-them out).

Thanks for any help.