[Mimedefang] Weird issue

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Please note that "Received:" may not be the only header that has multiple instances, although it is the most common.  However, if you don't do anything with any other multi-instance header, then there's nothing to worry about (unless you rewrite the message in its entirety from your storage of the headers).

Also note that this will not pick up headers that MD added at earlier stages, and sometimes, headers added by other milters.