Summary
This article provides a workaround to the limit of accounts that can be added to Outlook. By default, Outlook 2010 and Outlook 2013 allow up to 10 Exchange accounts to be configured in a single mail profile. If you for some reason need to add more than 10 Exchange accounts to your mail profile, you can extend this limit to via a Windows Registry tweak.
Body
When you try to add a shared mailbox to your Outlook, you received the following error message:
"Additional Microsoft Exchange Server accounts cannot be added. The maximum allowed number of Exchange accounts has been reached."
By default, Outlook desktop app allow up to 10 Exchange accounts to be configured in a single mail profile. If you for some reason need to add more than 10 Exchange accounts to your mail profile, you can extend this limit to via a Windows Registry tweak.
NOTE: The steps below should only be followed by I.T. Staff with experience in working with Windows Registry.
Increasing the limit to 9999 Exchange accounts
To increase the limit to up to 9999 Exchange accounts, the following Registry value needs to be created.
Key: HKEY_CURRENT_USER\Software\Microsoft\Exchange
Value name: maxnumexchange
Value type: REG_DWORD
Note 1: When you are using Outlook 2013, it is also recommended to set the below Registry key to prevent some crashing issues. You can then also increase the above value to up to 370000.
Increasing the limit to 370000 Exchange accounts
To increase the above limit to up to 370000 Exchange accounts in Outlook 2013, you’ll need to apply Hotfix KB2889951 or a later update. In addition, you must set the following Registry value as well.
Key: HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook
Value name: UseLargeEMSInstCtxHeap
Value type: REG_DWORD
Value data: 1