Be very sure that your server is not vulnerable to being used to relay third party email, since this is sure to attract spammers. One way of testing it is to telnet to relay-test.mail-abuse.org from the host in question:
% telnet relay-test.mail-abuse.org Trying 168.61.4.13... Connected to cygnus.mail-abuse.org. Escape character is '^]'. ... System appeared to reject relay attempts Connection closed by foreign host.
It is normal that you will see multiple entries in the "Checking for rejected mail hosts" section of the daily run output after running the above test.