Virus?

We have exchange 5.5 running on nt serv. 4 The past couple months we have had some rogue emails coming through adressed to some random array of letters and numbers (fd76fs@hotmail.com) note:it is always at hotmail.com Anyways I find these in the message que in the mornings. The originater is always <> real messages are in the format of And the messages are always very atrocious. Something like animal… you can quess. So I would really like to get rid of this. Relaying is block so this is not it. Any suggestions? Thanks in advance.