“Your Lotus Domino environment is unstable after upgrading to Domino 8.5.2 FP3. The server crashes every hour or so on the nIMAP task. This issue is an IMAP regression from 8.5.2 FP2 to 8.5.2 FP3.”
Link: Domino Server crashes on nIMAP after upgrading to 8.5.2 FP3