Inbound Mail Failure

My question is how can i find out were this Inbound mail Failure is coming from I’m currently running NT 4.0 service pack 6 with exchange 5.5 service pack 4.0. I would like to know if my server is the one with the problem. I got about 1000 of these notifications yesterday

MSEXCH:IMS:Merit Abrasives:nt_mis:NT_MIS 0 (000C05A6) Unknown Recipient
astraitfan@earthlink.net
MSEXCH:IMS:Merit Abrasives:nt_mis:NT_MIS 0 (000C05A6) Unknown Recipient
astraitnut@earthlink.net
MSEXCH:IMS:Merit Abrasives:nt_mis:NT_MIS 0 (000C05A6) Unknown Recipient
astraiton@earthlink.net

Not sure what I’m suppose to do to fix the problem.