What does a "550 Possible UBE/UCE detected - If you receive this message in error, please contact spamguard@mi-co.net for assistance" message mean?

The outgoing mail server that the sender is using has been listed as a source of spam in our database. There are two common ways the address is listed. The first involves servers that intentionally and knowingly allow their users to send spam (UCE or UBE). These will remain listed. The second involves servers that have been compromised by an outside source or virus and are being used to send out emails (frequently without the owner's knowledge or permission). Have the sender contact their technical support people for assistance. Once the problem has been addressed, the address will be removed from the database.
If you would like for us to examine the message, please cut and paste the error message headers and send them to tech support by clicking here to contact tech support.