During a particularly busy time on the server, a single user received so much email from Hotmail/Outlook at once that it effectively caused Microsoft to DDOS attack the server. Seems to be resolved now.
Can’t tell if Dovecot processes are the cause or a symptom. Heavy average usage may be masking the actual cause of the event. Restarting Dovecot, may see a few IMAP connections cut and then work again soon after.
Server load over 200, investigating cause
Seeing poor performance/responsiveness on Ocean server