If you’re unable to restart IMAP/POP3 Server (Courier-IMAP), have a look at the logs at /var/log/messages :
Following errors were found while digging the logs
Jul 29 22:42:21 asap3d courier-imap: Starting pop3 succeeded
Jul 29 22:42:21 asap3d pop3d-ssl.rc: bind: Address already in use
Jul 29 22:42:21 asap3d pop3d-ssl.rc: ll_daemon_start: Resource temporarily unavailable
Above error will pop in due to incorrect courier-pop3d configuration or courier-pop3d is enabled via xinetd on the server.
If you find courier-pop3d and courier-pop3ds configured via xinetd, do disable it by editing the files under /etc/xinetd.d. Once this is done, restart xinetd and then try restarting IMAP/POP3 Server (Courier-IMAP) from Plesk Service Management. It should work fine.
In the following example, check line number 3 (disable = yes) which disables the service under xinetd.
service pop3
{
disable = yes
flags = NODELAY
socket_type = stream
wait = no
user = root
server = /usr/bin/tcp-env
server_args
= -R -f /usr/lib/courier-imap/etc/pop3d /usr/lib/courier-imap/sbin/pop3login /usr/lib/courier-imap/libexec/authlib/authpsa /usr/lib/courier-imap/bin/pop3d
Maildir
log_type = FILE /var/log/pop3d.log
log_on_success = PID HOST EXIT DURATION
log_on_failure = HOST ATTEMPT
}
