[Mimedefang] smfi_chgheader returned MI_FAILURE

[Mark London]

Hi - I have mimedefang-2.54-1.2.el4.rf running on RHEL4 with 
spamassassin.  Once specific piece of mail is causing the following 
error message, preventing it from being delivered.

Apr 21 12:28:47 psfcsv1 mimedefang[28925]: l3LGRYZX001251: 
smfi_chgheader returned MI_FAILURE

How can I debug this problem?  We've had this configuration for many 
months, so it's only this one piece of mail which is causing a problem.  
Thanks. - Mark