Hi, i've got the same question / error.
From Microsoft side there isnt a way to increase the maximum of MAPI connection to a single mailbox. Unfortunately in some scenarios this is a must have!!
Hope this could be fixed or could be managed in some ways.
Answer from MS-Lead-Team:
There's no official limitation on that from Microsoft's site for most Outlook features.
So there's a soft limit of 20 MAPI connection and it can happen that you can't open the folders of the shared mailbox anymore.
(Soft limit because it doesn't happen when reach exactly 20) That limit can't be increased.
As the customer says it stops synchronizing, I would suggest to count the folders and subfolders of all added mailboxes to an user together and share the result with us.
Because there's a limitation of (around) 500 folders per Outlook session, that actually stops the synchronization in cache mode, and with 4 additional mailboxes added, it's more likely they hit that limit.
That's also a soft limit because it can happen with less than 500 (like 450) sometime it doesn't happen with more. It's depended on the local machine and the internet connection as well.
Next Answer:
There's no way to raise this limit.
The solution is to reduce the amount of folders or leave Exchange cache mode turned off.
