Repeated emails downloaded again and again

[Fernando SFEIR]

In my mail client, if setting the "Leave mail on server" to "send and receive" the mails from the inbox are downloaded again and again ...
This occurs when accessing through pop3 and imap.
In the mail downloaded, I can see that changing the UID (they increase each session).

A test I made:
fersfeir @ notebook: ~ $ telnet mail.mailserver.com.ar 110
Trying xx.xx.51.6 ...
Connected to mail.mailserver.com.ar.
Escape character is'^]'.
+ OK Hello there. <6699.1267578335 @ localhost.localdomain>
user [email protected]
+ OK Password required.
pass blahblah
+ OK logged in..
UIDL
+ OK
1 UID4-1267578183
.
quit
+ OK Bye-bye.
Connection closed by foreign host.
fersfeir @ notebook: ~ $ telnet mail.mailserver.com.ar 110
Trying xx.xx.51.6 ...
Connected to mail.mailserver.com.ar.
Escape character is'^]'.
+ OK Hello there. <6714.1267578357 @ localhost.localdomain>
user [email protected]
+ OK Password required.
pass blahblah
+ OK logged in..
UIDL
+ OK
1 UID5-1267578183
.
quit
+ OK Bye-bye.
Connection closed by foreign host.
fersfeir @ notebook: ~ $ telnet mail.mailserver.com.ar 110
Trying xx.xx.51.6 ...
Connected to mail.mailserver.com.ar.
Escape character is'^]'.
+ OK Hello there. <6727.1267578378 @ localhost.localdomain>
user [email protected]
+ OK Password required.
pass blahblah
+ OK logged in..
UIDL
+ OK
1 UID6-1267578183
.
quit
+ OK Bye-bye.
Connection closed by foreign host.
fersfeir @ notebook: ~ $
. com.ar 110
Trying xx.xx.51.6 ...
Connected to mail.mailserver.com.ar.
Escape character is'^]'.
+ OK Hello there. <6727.1267578378 @ localhost.localdomain>
user [email protected]
+ OK Password required.
pass blahblah
+ OK logged in..
UIDL
+ OK
1 UID6-1267578183
.
quit
+ OK Bye-bye.
Connection closed by foreign host.

I have installed Plesk 9.3.0 and courier:

dpkg-l | grep courier
ii psa-courier-imap-3.0.8 courier-imap debian5.0.build93091230.07
ii psa-mail-1.0.0-driver qc-mail debian5.0.build93091230.07 Plesk qmail-courier_imap driver

dpkg-l | grep psa
ii psa 9.3.0-debian5.0.build93091230.07 Plesk Interface and Utilities