After Plesk upgrade to 9.5.4, imapd on port 143 did not come up.
On checking the maillog, it was complaining about:
imapd: Invalid -maxprocsarg option.
imapd-ssl: Invalid -maxprocsarg option.
I noticed /etc/courier-imap/imapd had both the below lines commented:
Once I uncommented those values, I was able to get the imap listener up upon a `service courier-imap restart`.