My Datacenter guy called me about a month ago saying my machine was compromised.  He said he has been getting complaints from other carriers.  I logged in and didn't find anything that indicated it was compromised.  But when I went into the SmarterMail Spool I found a few large messages about 8MB+ each that all had the same message.  The word Failed:  Repeated over and over again.  It was sent from "System Administrator"

Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed: Failed:

It was a system sent message, so there was no record of it in the archive, or even the logs.  To me it looks like a mail loop of some kind.  The only way I was able to catch it was to enable delivery delay to 20 seconds giving me enough time to catch it in the spool.  Once I deleted it, the issue stopped. but then a few days later it started up again.  Now I am logging into my server every few days to look for these messages, so I can delete them.  Anyone have any idea what the cause could be?