Multiple Exchange accounts limit

By default, Outlook allows 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 up to 9999 via a Registry tweak.

With an additional Registry setting, you can even extend this limit to 370000 Exchange accounts in Outlook 2013 and later.

You’ll get a notification when you've reached the maximum amount of Exchange accounts.

Note: With a single Exchange account, you can again connect to multiple mailboxes that are hosted within the same Exchange Organization. These additional mailboxes do not count towards the accounts limit. You can see your amount of configured Exchange accounts via;
File-> Account Settings-> Account Settings…

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: When you are using Outlook 2013 or later, it is recommended to also set the below Registry key to prevent some crashing issues. You can then even 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.

Outlook 2016, Outlook 2019 and Outlook as part of a Microsoft 365 subscription do not need a specific update for this.

For either version, you must set the following Registry value as well.

Key for Outlook 2013;
HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook

Key for Outlook 2016, 2019, and Microsoft 365;
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook

Value name: UseLargeEMSInstCtxHeap
Value type: REG_DWORD
Value data: 1

Administrators: Group Policy

The same setting is also controlled via the following Group Policy;

User Configuration-> Administrative Templates-> Microsoft Outlook <version>-> Account Settings-> Exchange-> Set maximum number of Exchange accounts per profile

This Group Policy “only” allows you to set a maximum of 9999 Exchange accounts per profile.