X-UID in mail header is removed?

Dear Dovecot Mailing List,

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

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).

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@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@localhost.localdomain; Thu, 28 May 2015 13:39:11 +0900 Received: (from root@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@localhost.localdomain Message-Id: 201505280439.t4S4dB6u006056@localhost.localdomain To: test@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@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@localhost.localdomain; Thu, 28 May 2015 11:07:42 +0900 Received: (from root@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@localhost.localdomain Message-Id: 201505280207.t4S27fXd007197@localhost.localdomain To: test@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@localhost.localdomain X-Original-To: test Delivered-To: test@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@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@localhost.localdomain From: root@localhost.localdomain (root)

test . UIDL +OK 1 0000000155652c77

Thanks, Masaharu Kawada