X-UIDL question

["John Fleming" <john@wa9als.com>]

I've switched from unstable (via Knoppix) to testing.  I'm using Postifx as
before.  The last line in my mail headers is an X-UIDL header (see below)
that I hven't seen before.  Any idea where this is coming from and how I can
affect it?  Thanks - John

X-Virus-Status: No
X-Virus-Checker-Version: clamassassin 1.2.1 with clamdscan / ClamAV version
0.75-1 signatures 24.456
X-Spam-Checker-Version: SpamAssassin 2.63 (2004-01-11) on Luke.wa9als.com
X-Spam-Level:
X-Spam-Status: No, hits=0.0 required=5.0 tests=none autolearn=no
version=2.63
X-UIDL: Z<W!!BgU!!=G]!!KJ_!!