i don’t know if its an update on microsofts part with outlook or with something updated on Nethserver but recently i have noticed a couple of different email mailboxes not syncing correctly.
This is using the IMAP connection and sometimes a mailbox that is being used on a number of installs of outlook.
what happens is a user reads an email so the email can be seen as read, another person on a different computer even after a send and receive the email is still un-read.
i have also experienced this on my laptop and my desktop pc.
i have cleared the cache in outlook on both machines and it still seems to do it.
While it generally works, it has had a LOT of issues in the past 20+ years. The Exchange connector is better debugged.
To “isolate” the issue, you can install eg Thunderbird next to Outlook, also using IMAP (Even on both machines!). In my experience, TB handles IMAP MUCH better than Outlook. If the issue also persists on TB, it’s probably a NS issue, if not, its a MS issue…