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Possible network related error when trying to open Exch2016 mailboxes

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For some reason, a user who has full access to all mailboxes is unable to expand other users boxes in Outlook view. Also his mailbox doesn't always open. When it is not in cached mode an error is displayed:
Non cached error when opening outlook
In cached mode the mailbox opens but usually complains that it is disconnected. Connecting to Exchange fails until it suddenly succeeds.

The problem is very similar as in this topic: https://www.experts-exchange.com/questions/29125111/Intermittently-cannot-open-Outlook-The-set-of-folders-cannot-be-opened-Network-problems-are-preventing-connection-to-Microsoft-Exchange.html 
However in my case the mailbox migration to a different database didn't help.

Our on premises Exchange environment was migrated from Exchange 2010 to 2016 and after that these problems have followed users occasionally. Setting a registry value to prefer IPV4 over IPV6 brought some stability but even after that we've had some reports of these problems. I have a feeling that the problem might be somehow MAPI related and it has to do with the amount of mailbox objects etc.. I know there is a registry value that is mentioned in some discussions that can be raised but others are against messing with Exchanges registry.

Now the full access user and it's mailbox is used for a software that needs to sync data from other users mailboxes. When the sync process tries to run an error is displayed:
Software User: username
PIM Account: username
Error Message: A MAPI related error occurred (MAPI_E_NETWORK_ERROR). Component = Microsoft Exchange Information Store, Description = Network problems are preventing connection to Microsoft Exchange.
Error Location: IDC_MAPISession::LogonInternalUsingSeparateProfile

Also this topic has similar problem: https://community.spiceworks.com/topic/2162356-outlook-exchange-randomly-disconnects-users 

Any suggestions on what to check?

Edit. We only have one Exchange server that holds all roles. Exchange 2016 with CU 15. All clients are Win10 x64 with Office 2016 x64
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AlexA lack of information provides a lack of a decent solution.

Commented:
Hey ya,

How many mailboxes are currently connected to his outlook client? I think there is a limit of between 6 and 10 before the client starts crashing out.

Thanks
Alex
I TCIO

Author

Commented:
Probably about a 100 :)

I found this: https://www.experts-exchange.com/questions/29113895/Some-mailboxes-Disconnect-randomly-after-migration-from-Exchange-2010-to-2016.html?searchSuccess=true&searchTop10=true 

So I will give it a go on our admin mailbox and see if the throttling policy makes any difference.
AlexA lack of information provides a lack of a decent solution.

Commented:
Yeah, that's never going to work, it's not even supported.

Change the mailbox so it doesn't automatically add to his client and then tell him to either open each one manually or use OWA.

Regards
Alex
I TCIO

Author

Commented:
Throttling policy did make a difference and now the processes are running successfully. That makes me wonder if the default policy settings are too low for our environment, thus causing connectivity problems. We never had this kind of problems with Outlook 2010 + Exchange 2010.
CIO
Commented: