netbsd-help: Does anyone know what the X-UIDL mail header is?

Subject: Does anyone know what the X-UIDL mail header is?
To: 'netbsd-help' <netbsd-help@NetBSD.ORG>
From: John Maier <JohnAM@datastorm.com>
List: netbsd-help
Date: 08/20/1996 14:45:00

I've seen this hidden among the subject and from lines of some of my 
emails, what is it?
thanks
jam