Since one of our customers updated their server courier does not handle IMAP connections properly any more. POP3 works without any problems.

When I try to test IMAP with telnet then it is always like this:

$ telnet domain.com 143
Trying 188.40.46.214...
Connected to domain.com.
Escape character is '^]'.
* OK [CAPABILITY IMAP4rev1 UIDPLUS CHILDREN NAMESPACE THREAD=ORDEREDSUBJECT THREAD=REFERENCES SORT QUOTA IDLE ACL ACL2=UNION STARTTLS] Courier-IMAP ready. Copyright 1998-2011 Double Precision, Inc.  See COPYING for distribution information.
01 LOGIN [email protected] test
Connection closed by foreign host.

I enabled debugging in the authdaemond but the output does not really help much:

Apr 12 23:10:04 servername authdaemond: received auth request, service=imap, authtype=login
Apr 12 23:10:04 servername authdaemond: authmysql: trying this module
Apr 12 23:10:04 servername authdaemond: SQL query: SELECT login, password, "", uid, gid, homedir, maildir, quota, "", concat('disableimap=',disableimap,',disablepop3=',disablepop3) FROM mail_user WHERE login = '[email protected]' 
Apr 12 23:10:04 servername authdaemond: password matches successfully
Apr 12 23:10:04 servername authdaemond: authmysql: sysusername=<null>, sysuserid=5000, sysgroupid=5000, homedir=/var/vmail, [email protected], fullname=<null>, maildir=/var/vmail/domain.com/test, quota=0, options=disableimap=n,disablepop3=n
Apr 12 23:10:04 servername authdaemond: Authenticated: sysusername=<null>, sysuserid=5000, sysgroupid=5000, homedir=/var/vmail, [email protected], fullname=<null>, maildir=/var/vmail/domain.com/test, quota=0, options=disableimap=n,disablepop3=n

Right after the "Authenticated" line the output stops. There is no other message. And in no other log file I've checked I could find any other related message.

The system was updated from Ubuntu 10.10 to 12.04.

How could I get more information? Or does anybody have an idea what could go wrong here?