Changes have been made in Git for this ticket:

Bug #10417: Fix regression in POP3 Socket driver that caused incorrect 
parsing of server data

  2 files changed, 9 insertions(+), 6 deletions(-)
http://git.horde.org/horde-git/-/commit/c697fa16ac93fe2cbfff292347b18323528bbdcf

Two Logs one from courier one from dovecot:

DebugLog Server1: [courier pop3 Server]
S (1312891582,22): +OK Hello there.
C (1312891582,22): CAPA
S (1312891582,24): +OK Here's what I can do:
S (1312891582,24): SASL PLAIN LOGIN CRAM-MD5 CRAM-SHA1 CRAM-SHA256
S (1312891582,24): TOP
S (1312891582,24): USER
S (1312891582,24): LOGIN-DELAY 10
S (1312891582,24): PIPELINING
S (1312891582,24): UIDL
S (1312891582,24): IMPLEMENTATION Courier Mail Server
S (1312891582,24): .
C (1312891582,24): USER mail@example.com
S (1312891582,26): +OK Password required.
C (1312891582,26): [USER Command - password]
S (1312891582,28): +OK logged in.
C (1312891582,3): QUIT
S (1312891582,32): +OK Bye-bye.

DebugLog Server2: [dovecot pop3 Server]
S (1312891864,23): +OK mDd-MDA ready.
C (1312891864,23): CAPA
S (1312891864,25): +OK
S (1312891864,25): CAPA
S (1312891864,25): TOP
S (1312891864,25): UIDL
S (1312891864,25): RESP-CODES
S (1312891864,25): PIPELINING
S (1312891864,25): STLS
S (1312891864,25): USER
S (1312891864,25): SASL PLAIN DIGEST-MD5 CRAM-MD5
S (1312891864,26): .
C (1312891864,26): USER netwalker@lanparty-nrw.de
S (1312891864,28): +OK
C (1312891864,28): [USER Command - password]
S (1312891866,77): +OK Logged in.
C (1312891866,79): QUIT
S (1312891866,82): +OK Logging out.

To further debug this issue, we need details of the IMP -> IMAP/POP 
communication.

To enable debugging, see instructions contained in 
imp/config/backends.php (the 'debug' config parameter).

Debugging should not be enabled on a production server,   Attach/post 
only the portion of the log that directly deals with the problem 
reported (it may be simplest to clear the log file and then perform 
the event that causes the error).

POP3 Connection don't work in current release! Server Supports UIDL 
but IMP check says no!

I checked the pop3 feature with dovecot pop3 and courier pop3 Server 
both Support the UIDL Feature. If I try with telnet both tell me after 
the CAPA command UIDL is Supported

IMP5 currently don't work with pop3!?