X-UID in mail header is removed?

[mkawada at redhat.com]

Dear Dovecot Mailing List,

I have 2 questions regarding dovecot and would very much appreciate if 
you would kindly give me helps.

<Question 1>
Is 'X-UID' no longer shown in mail header via pop3? I have checked with 
the following 3 version of dovecot packages in order to make sure if a 
message sent by dovecot have 'X-UID' in its mail header, and the result 
is that only "dovecot-0.99.11-10.EL4" shows 'X-UID' and the other ones 
do not(Please see the result written in the second half of this mail).

<Question 2>
Is 'X-UID' used for checking if a message has already been sent to a 
user so that he never gets any duplicated mails that are already in his 
mbox? The reason why I am asking this here is that I am working on a 
problem that a user sometimes gets a duplicated mail from dovecot if the 
user keeps mails on the server as its settings. I actually thought this 
kind of thing would happen if the 'UIDL' gets corrupted or changed for 
some reasons. However, it is said that this issue never happened in 
using "dovecot-0.99.11-10.EL4" which shows 'X-UID' in mail header and in 
using "dovecot-2.0.9-8.el6_6.4" which does not show 'X-UID', the issue 
happens sometimes. Thus, I would like information indicating what is 
exactly used to avoid getting any duplicated messages while downloading 
a message via pop3 from dovecot.


Here is the test result I checked:

(dovecot-0.99.11-10.EL4)
]# telnet localhost 110
Trying 127.0.0.1...
Connected to localhost.localdomain (127.0.0.1).
Escape character is '^]'.
+OK dovecot ready.
user test
+OK
pass password
+OK Logged in.
list
+OK 1 messages:
1 786
.
RETR 1
+OK 786 octets
Return-Path: <root at localhost.localdomain>
Received: from localhost.localdomain (localhost.localdomain [127.0.0.1])
     by localhost.localdomain (8.13.1/8.13.1) with ESMTP id t4S4dBcu006057
     for <test at localhost.localdomain>; Thu, 28 May 2015 13:39:11 +0900
Received: (from root at localhost)
     by localhost.localdomain (8.13.1/8.13.1/Submit) id t4S4dB6u006056
     for test; Thu, 28 May 2015 13:39:11 +0900
Date: Thu, 28 May 2015 13:39:11 +0900
From: root <root at localhost.localdomain>
Message-Id: <201505280439.t4S4dB6u006056 at localhost.localdomain>
To: test at localhost.localdomain
Subject: test mail
X-IMAPbase: 1432787986 1
Status: O
X-UID: 1 <--X-UID shown here
Content-Length: 5
X-Keywords:

test
.
UIDL
+OK
1 1432787986.1


(dovecot-1.0.7-9.el5_11.4)
]# telnet localhost 110
Trying 127.0.0.1...
Connected to localhost.localdomain (127.0.0.1).
Escape character is '^]'.
+OK Dovecot ready.
user test
+OK
pass password
+OK Logged in.
list
+OK 1 messages:
1 606
.
RETR 1
+OK 606 octets
Return-Path: <root at localhost.localdomain>
Received: from localhost.localdomain (localhost.localdomain [127.0.0.1])
     by localhost.localdomain (8.13.8/8.13.8) with ESMTP id t4S27gBo007199
     for <test at localhost.localdomain>; Thu, 28 May 2015 11:07:42 +0900
Received: (from root at localhost)
     by localhost.localdomain (8.13.8/8.13.8/Submit) id t4S27fXd007197
     for test; Thu, 28 May 2015 11:07:41 +0900
Date: Thu, 28 May 2015 11:07:41 +0900
From: root <root at localhost.localdomain>
Message-Id: <201505280207.t4S27fXd007197 at localhost.localdomain>
To: test at localhost.localdomain
Subject: testmail

test
.
UIDL
+OK
1 0000000155667888
.

(dovecot-2.0.9-8.el6_6.4)
]# telnet localhost 110
Trying ::1...
telnet: connect to address ::1: Connection refused
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
+OK Dovecot ready.
user test
+OK
pass password
+OK Logged in.
list
+OK 1 messages:
1 605
.
RETR 1
+OK 605 octets
Return-Path: <root at localhost.localdomain>
X-Original-To: test
Delivered-To: test at localhost.localdomain
Received: by localhost.localdomain (Postfix, from userid 0)
     id 3629F4336A; Wed, 27 May 2015 11:34:39 +0900 (JST)
Date: Wed, 27 May 2015 11:34:39 +0900
To: test at localhost.localdomain
Subject: test mail
User-Agent: Heirloom mailx 12.4 7/29/08
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Message-Id: <20150527023439.3629F4336A at localhost.localdomain>
From: root at localhost.localdomain (root)

test
.
UIDL
+OK
1 0000000155652c77

Thanks,
Masaharu Kawada