Hi Andy,
How did you change the notification sound to a custom one? Please try the following method and then see if you get what you want:
1. Make sure notification options in Outlook and Windows Notification settings are both turned on.


2. Open Control Panel and open Sound, switch to Sounds tab.
3. try changing the sound for "Desktop Mail Notification" option to None.

4. Scroll down to the "New Mail Notification" option and click Browse button to change it to the custom sound.

5. Click OK. Try sending yourself a new email to see the result.
Regards,
Steve Fan
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