PROBLEM:
While Login into Webmail it returns the error: “Login failed”

SOLUTION:
Make sure that it is possible to connect to localhost:143 from the command line using telnet. If you get the error as below:
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# telnet localhost 143
localhost/143: Name or service not known
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then check /etc/hosts file. It must have permissions -rw-r–r– and contain the record for localhost in the following format:
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127.0.0.1       localhost localhost.localdomain
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Above is from From Parallels Knowledgebase

If /var/log/psa-horde/psa-horde.log shows something like:

FAILED LOGIN xx.xx.xx.xx to localhost:143[imap/notls] as me@mydomain.co.uk
FAILED LOGIN xx.xx.xx.xx to localhost:143[imap/notls] as me@mydomain.co.uk

I have searched regarding this but unable to find any fix so, I just re-installed the IMAP Service on the server. Following are the steps using which I have re-installed IMAP Server:
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1) You can find the RPMS used by Plesk in /root/psa/PSA_version directory OR you can download the Tar (Not Autoinstaller) containing RPMS from Parallels website http://www.parallels.com/en/download/ (need registration it’s free)

2) go to the RPM directory which contains courier Imap RPM’s
#cd [path to the PSA RPM Directory]/dist-rpm-CentOS-4.2-i386/base/

3) Re-install courier-imap-3.0.8-cos4.build81070322.16.i586.rpm and psa-courier-imap-add-8.1.1-cos4.build81070322.16.i586.rpm which will automatically restart the services too
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[root@vps base]# pwd
/root/plesk/dist-rpm-CentOS-4.2-i386/base
[root@vps base]# rpm -Uvh courier-imap-3.0.8-cos4.build81070322.16.i586.rpm psa-courier-imap-add-8.1.1-cos4.build81070322.16.i586.rpm –force
Preparing… ########################################### [100%]
Reloading configuration: [ OK ]
1:courier-imap ########################################### [ 50%]
Stopping Courier-IMAP server:
Stopping imap [ OK ]
Stopping imap-ssl [ OK ]
Stopping pop3 [ OK ]
Stopping pop3-ssl [ OK ]

Starting Courier-IMAP server:
Starting imapd [ OK ]
Starting imap-ssl [ OK ]
Starting pop3 [ OK ]
Starting pop3-ssl [ OK ]

2:psa-courier-imap-add ########################################### [100%]
Stopping Courier-IMAP server:
Stopping imap [ OK ]
Stopping imap-ssl [ OK ]
Stopping pop3 [ OK ]
Stopping pop3-ssl [ OK ]

Starting Courier-IMAP server:
Starting imapd [ OK ]
Starting imap-ssl [ OK ]
Starting pop3 [ OK ]
Starting pop3-ssl [ OK ]

[root@vps base]# netstat -nap | grep :143
tcp 0 0 0.0.0.0:143 0.0.0.0:* LISTEN 1657/couriertcpd
[root@vps base]# telnet localhost 143
Trying 127.0.0.1…
Connected to localhost.
Escape character is ‘^]’.
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