Exchange sever

HI ,

My exchange server was working fine until errors appearing in event viewer as follow :

The description for Event ID [ 0 ] in Source [ Pop3GateWay ] cannot be found. The local computer may not have the necessary registry information or message DLL files to display message from remote computer. The following information is part of the event: Pop3GateWay error: 87, – SendMail ? Bad To:.

Can anyone tell where did tell error come from ?