20 x client on LAN with Thunderbird (lets not discuss this choice) connecting to the Exchange server via IMAP (STARTTLS)
One of the workstations has a issue were several times a day thunderbird report 'The connection was refused'. Restarting the workstation sometimes solves this.
The event logs on the workstation do not show anything related.
The event logs on the server do not show anything related.
I don't have much experience with IMAP on Exchange - Maybe there is some tweak to make it work 'correct' ?