As usually there might be a lot of reasons for that. In my case the reason became clear when I checked /var/log/messages:
imapd-ssl: /etc/courier-imap/shared/index: No such file or directory problem. The quick and dirty solution appeared to be really quick.
touch /etc/courier-imap/shared/index and the problem will be gone. I didn’t do root cause analysis for this particular case. The macmail was not set up to retrieve shared folders, possibly it does that just by default. But the problem is really gone with this fix. The configuration without /etc/courier-imap/shared/index seems to be default for systems managed with Plesk 8.2
This post is published in Linux, Other.