Categories synchronization between Outlook Web Client and Outlook Desktop (MacOS) client not working

Hi @Ayoub Chouak

May I ask did you meet this issue when you create a new category or assign a category for a message?

For both scenarios, I did some testing on version 16.108.1, and I did not see the same behavior as yours.

The newly created category showed up right away. The same for assigning a category.

So first, you can try updating your New Outlook for Mac. You can also try to remove and re-add account to force a resync.

If the issue persists, consider reinstall New Outlook for Mac via Update history for Office for Mac - Office release notes | Microsoft Learn.

In this page, you can download the latest version 16.108.1, or if you do not see this behavior previously, you can download older version such as 16.104.x.

Please try and let me know if it works for you. I look forward for your reply.

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Category sync delays between Outlook on the web and Outlook for Mac are typically caused by issues with the Master Category List (MCL) stored in the mailbox on the Exchange server. When the MCL is corrupted, Outlook for Mac can show errors or behave inconsistently with categories and may not reflect changes promptly.

A supported workaround is to reset the MCL via Outlook on the web, then recreate any missing custom categories. Note that this can reset categories to defaults and custom categories may be lost, so documenting them first is important.

Follow these steps:

1. Document existing categories
   1. In Outlook for Mac, select Categorize → Edit Categories.
   2. When the error or dialog appears, select OK so the list of categories is visible.
   3. Take a screenshot of the categories (for example, with Command+Shift+3) so the names and colors can be restored later.
2. Reset the Master Category List using Outlook on the web
   1. Sign in to Outlook on the web (Outlook Web App).
   2. Right‑click any email message, select Categorize, then Manage categories.
   3. Change the color of any category (click the arrow next to the color block and choose a different color).
   4. Select OK to save.
   5. Go back into Manage categories and change that category’s color back to the original.
   After this, the MCL is regenerated on the server. Some custom categories may disappear from the master list, but items that had those categories still retain the category names.
3. Restore missing custom categories in Outlook for Mac
   1. In Outlook for Mac, select Categorize → Edit Categories.
   2. Select the + button to add a new category.
   3. Enter the exact name of each missing custom category, using the screenshot as a reference.
   4. Choose the appropriate color for each category.
   5. Select Add and repeat for all missing categories.

After the MCL is reset and categories are re‑added, category information should be consistent between Outlook on the web and Outlook for Mac, and new category changes should sync more reliably.

References:

• "Outlook can’t reach the server to get the categories" error in Outlook 2016 for Mac when you edit categories