I had a similar setup, and what helped me troubleshoot the IMAP traffic was doing packet captures on both the MX and the Exchange server. Also double-checked firewall rules on both ends. Make sure your NAT rules aren’t conflicting with any outbound rules. Found some useful examples on acupowererp.com that showed how to structure rules for this kind of hybrid flow. ... View more

I had to allow IMAP (port 143 or 993) through the firewall to the Exchange server and make sure the server was accepting connections from the CRM’s IP range. ... View more