We have an Exchange Environment with 2 X Exchange 2016 servers and 1 X Exchange 2019. EX2016 is running on Server 2016 and EX2019 on server 2019. One issue we are experiencing is when Outlook clients connect via the EX2019 server and their mailbox resides on the EX2019 server, the mailbox opens fine. If they are then opening shared mailboxes or mailboxes with delegated access which reside on either of the Exchange 2016 servers, they are not able to view the mailboxes. The Outlook connection status just shows as 'Connecting' and an 'Error' status under Authn.
If they connect via either of the Exchange 2016 servers, all works completely fine!
As testing on this, I am simply connecting from a remote PC running Outlook 365 and changing the NAT rule to point to either the EX2019 server (which doesn't work) or the EX2016 server (which does work).
As far as I can see all settings on virtual DIRs are identical.
ExcludeExplicitO365Endpoint added to reg
Has anyone experienced this issue or got any suggestions, please?
Thank you in advance.