- Official Post
Sometimes people need to open more than 10 mailboxes in Outlook but this is not possible due to a limit of the default Outlook settings.
With a simple registry entry you can change this.
Code
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\Microsoft\Exchange]
"maxnumexchange"=dword:00000015
But I wouldn't change this to a too high value - 15/20 and than you should consider in having 2 or more Outlook profiles.