Just a heads up as this has caused me some problems and may help others. I have an old ISPConfig 3 server that's been running for a few years now and uses Courier rather than Dovecot. I updated to the latest release ISPConfig 3 a short while ago and thought everything was OK until I got an email from one of my resellers yesterday. Any new mailboxes added since the upgrade aren't functioning correctly. Any attempt to connect to them via pop3 produces the error... -ERR Maildir invalid (no 'cur' directory) It would appear that as part of the upgrade the mail configuration had been changed to dovecot from courier. Not sure if this is a new configuration option that has been added or the existing one that was changed as part of the upgrade. Changing the server configuration to Courier and recreating the bad mailboxes has fixed the problem. The other servers I upgraded at the same time were running dovecot anyway so it wasn't an issue on those.