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At times there are certain services that still exist however they are not running (dead), in such cases the service needs to be killed and restarted. I have found this to be the case when Plesk is upgraded between versions and mail has stopped running. The following steps quickly rectify the issueStep 1: Check the status of the service "courier-imapd"
[root@*****~]# service courier-imapd status
courierlogger dead but pid file exists
Step 2: Find the PID number for the service
[root@*****~]# ps aux | grep courierlogger
root 2807 0.0 0.0 103316 856 pts/0 S+ 10:32 0:00 courier-imapd
Step 3: Kill the service using the PID number from the above output[root@*****~]# kill -9 2807
Step 4: Restart the mail services
[root@*****~]# /etc/init.d/courier-imapd start
Starting Courier IMAP server: [ OK ]
[root@*****~]# /etc/init.d/courier-pop3d start
Starting Courier POP3 server: [ OK ]
[root@*****~]# /etc/init.d/courier-imaps start
Starting Courier IMAP server with SSL/TLS support: [ OK ]
[root@*****~]# /etc/init.d/courier-pop3s start
Starting Courier POP3 server with SSL/TLS support: [ OK ]
After this the mail should start working. If there are any further issues, please raise a ticket with as much information as possible
including errors and screenshots where possible and our support teams will assist you further.