How to Fix Outlook 2021 Not Adding an O365 Mailbox

Learn how to troubleshoot and resolve the issue of Outlook 2021 not adding an O365 mailbox on a Windows 10 PC.

Problem

If you are using Outlook 2021 on a Windows 10 PC, you may encounter a problem where you cannot add a specific O365 mailbox to your Outlook profile. This can be frustrating and confusing, especially if you can add other O365, Hotmail, or Gmail accounts without any issues. In this article, we will explain why this problem occurs and how to fix it with some simple steps.

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What Causes Outlook 2021 Not to Add an O365 Mailbox?

There are several possible reasons why Outlook 2021 may not be able to add an O365 mailbox to your profile. Some of the common ones are:

• The O365 mailbox is already added to another Outlook profile on the same PC. Outlook does not allow adding the same O365 mailbox to multiple profiles on the same PC.
• The O365 mailbox has a different primary email address than the one you are using to sign in to Outlook. Outlook requires that the primary email address of the O365 mailbox matches the one you use to sign in to Outlook.
• The O365 mailbox has some corrupted or outdated settings that prevent Outlook from connecting to it. Outlook may fail to add the O365 mailbox if there are some errors or conflicts in the mailbox settings, such as the autodiscover service, the authentication method, or the server name.
• The Outlook profile or the Windows registry has some corrupted or outdated entries that prevent Outlook from adding the O365 mailbox. Outlook may not be able to add the O365 mailbox if there are some issues with the Outlook profile or the Windows registry, such as the maximum number of Exchange accounts, the use of large EMS instance context heap, or the Outlook security settings.

How to Fix Outlook 2021 Not Adding an O365 Mailbox?

Depending on the cause of the problem, there are different solutions that you can try to fix Outlook 2021 not adding an O365 mailbox. Here are some of the most effective ones:

Solution 1: Remove the O365 mailbox from any other Outlook profile on the same PC

If you have added the O365 mailbox to another Outlook profile on the same PC, you need to remove it before you can add it to your current profile. To do this, follow these steps:

1. Open Outlook and select the File tab on the ribbon, then select Account Settings > Account Settings.
2. Select the Email tab and highlight the O365 mailbox that you want to remove, then choose Remove.
3. Choose Yes to confirm the removal, then close the Account Settings window.
4. Restart Outlook and try to add the O365 mailbox again.

Solution 2: Use the correct primary email address of the O365 mailbox to sign in to Outlook

If you are using a different email address than the primary one of the O365 mailbox to sign in to Outlook, you need to change it to the correct one. To do this, follow these steps:

1. Sign in to the O365 portal with your O365 credentials and go to the Settings page.
2. Under Your app settings, select Mail, then select Account > My account.
3. Under Email account, note the primary email address of your O365 mailbox, which is the one that ends with @yourdomain.onmicrosoft.com.
4. Open Outlook and select the File tab on the ribbon, then select Account Settings > Account Settings.
5. Select the Email tab and highlight your O365 account, then choose Change.
6. Under User Information, change the E-mail Address field to the primary email address of your O365 mailbox that you noted earlier, then choose Next > Finish > Close.
7. Restart Outlook and try to add the O365 mailbox again.

Solution 3: Update the settings of the O365 mailbox to match the Outlook requirements

If there are some errors or conflicts in the settings of the O365 mailbox, you need to update them to match the Outlook requirements. To do this, follow these steps:

1. Sign in to the O365 portal with your O365 credentials and go to the Admin center.
2. Under Users, select Active users, then select the O365 mailbox that you want to add to Outlook, then choose Edit next to Mail settings.
3. Under Email apps, make sure that Outlook desktop is enabled, then choose Save changes.
4. Under Email connectivity, make sure that the authentication method is set to User name and password, the server name is set to outlook.office365.com, and the autodiscover service is enabled, then choose Save changes.
5. Restart Outlook and try to add the O365 mailbox again.

Solution 4: Modify the Outlook profile or the Windows registry to allow adding the O365 mailbox

If there are some issues with the Outlook profile or the Windows registry, you need to modify them to allow adding the O365 mailbox. To do this, follow these steps:

1. Close Outlook and open the Run dialog box by pressing Windows + R keys on your keyboard.
2. Type regedit and choose OK to open the Registry Editor.
3. Navigate to the following key: HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook
4. Right-click on the Outlook key and choose New > DWORD (32-bit) Value.
5. Name the new value UseLargeEMSInstCtxHeap and set its value data to 1, then choose OK.
6. Navigate to the following key: HKEY_CURRENT_USER\Software\Policies\Microsoft\Exchange
7. Right-click on the Exchange key and choose New > DWORD (32-bit) Value.
8. Name the new value MaxNumExchange and set its value data to the number of O365 mailboxes that you want to add, then choose OK.
9. Close the Registry Editor and restart your PC.
10. Open Outlook and try to add the O365 mailbox again.

Frequently Asked Questions (FAQs)

Question: How do I check the version of Outlook that I am using?

Answer: To check the version of Outlook that you are using, open Outlook and select the File tab on the ribbon, then select Office Account > About Outlook. You will see the version number and the build number of your Outlook.

Question: How do I check the version of Windows that I am using?

Answer: To check the version of Windows that you are using, open the Settings app and select System > About. You will see the edition, the version, and the build number of your Windows.

Question: How do I backup and restore the Windows registry?

Answer: To backup and restore the Windows registry, follow these steps:

1. Open the Registry Editor by following the steps mentioned earlier.
2. Select the key that you want to backup and choose File > Export.
3. Choose a location and a name for the backup file, then choose Save.
4. To restore the backup, choose File > Import and select the backup file, then choose Open.

Summary

In this article, we have explained why Outlook 2021 may not be able to add an O365 mailbox to your profile and how to fix it with some simple steps. We have also provided some FAQs related to the topic. We hope that this article has helped you resolve the issue and enjoy using Outlook 2021 with your O365 mailbox.

Disclaimer: This article is for informational purposes only and does not constitute professional advice. We are not responsible for any damages or losses that may result from following the instructions or using the tools mentioned in this article. Always backup your data and consult an expert before making any changes to your system.