odd cfmail / Undelivr problem CF9

The majority of mail is being sent out fine, but once or twice a week we'll have email, that is being sent from a cfmail tag to an internal email address that contains a customer message, that gets sent to the Undelivr folder.  Opening up the .cfmail file and inspecting the contents, everything looks fine.  If I move the file to the spool dir, it gets kicked right back to the Undelivr dir.  There is no record on the mail server log files for this email and the coldfusion mail.log file simply says:

"Error","scheduler-0","10/08/10","13:26:59",,""   Which is blank where the error details should be.

I've tried changing the subject of the email and sending it again and it gets moved right back to the Undelivr folder.  Our web system has a built in 2nd email reminder function that sends out the same email again if the 1st one didn't receive a reply after a week.  The 2nd email will often get received, which tips us off that the initial email must be stuck in the Undelivr dir.

Any ideas?  Thanks!