problem with pop3d, messages can be downloaded multiple times

Joseph Brennan brennan at
Wed Dec 7 09:29:30 EST 2005

Some more on this.

When a pop client does "leave on server", it has to compare what's on
the server with a database it maintains locally.  The key is something
called UIDL, a unique identifier per message maintained by the server.
The client sends a UIDL command and gets back a list of UIDLs.

This report <>
alludes to Outlook 2002 not handling the case where UIDLs are of varying
length.  However they are allowed to be varying length.  The standard
(RFC 1939) allows length of "one to 70 characters" and the example given
in RFC 1939 actually shows two UIDLs of different lengths:

          C: UIDL
          S: +OK
          S: 1 whqtswO00WBw418f9t5JxYwZ
          S: 2 QhdPYR:00WBw1Ph7x7

In the response each line has message number, space, UIDL.

U Wash seems to give UIDLs always of the same length (this is from
our U Wash pop3 server):

	+OK Unique-ID listing follows
	1 4304a6fc00000001
	2 4304a6fc00000002
	3 4304a6fc00000003
	4 4304a6fc00000004
	5 4304a6fc00000005
	6 4304a6fc00000006
	7 4304a6fc00000007
	8 4304a6fc00000008
	9 4304a6fc00000009
	10 4304a6fc0000000a
	11 4304a6fc0000000b
	12 4304a6fc0000000c

Cyrus gives UIDLs that can vary in length (our Cyrus server):

	+OK unique-id listing follows
	1 1125515674.1
	2 1125515674.2
	3 1125515674.3
	4 1125515674.4
	5 1125515674.5
	6 1125515674.6
	7 1125515674.7
	8 1125515674.8
	9 1125515674.9
	10 1125515674.10
	11 1125515674.11
	12 1125515674.13

So if Outlook cannot handle varying length, it breaks here, but the
standard definitely allows varying length.

I wonder whether this affects all recent versions of Outlook.

Joseph Brennan
Columbia University Information Technology

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