To cut a long story short, Domino IMAP functionality is very broken on Domino 8.5.0 – at least on the Linux platform we’re running.
Hitting our IMAP server from Mail.app after the upgrade resulted in an immediate crash of the server, which is easily reproducible – each and every connection causes the same Fatal Error. This is open as a PMR and already has been opened as an SPR – others have hit the same issue.
So, if you’re running Domino at 8.5beta2 or earlier with IMAP clients – do not upgrade just yet!!