Hi there!
With regards to the "Other" mailbox, are you referring to the Outlook mail application on Mac? If so, this can be disabled on the app itself. On the View tab in the Outlook app, depending on if you want to turn in on or off, select Turn on Focused Inbox or Turn off Focused Inbox.
When it's on, the Focused and Other tabs will appear at the top of your inbox. You’ll be informed about email flowing to Other, and you can switch between tabs any time to take a quick look. Apparently, the notifications itself cannot be turned off so you need to turn off the focused inbox feature as a whole.
You can also find more details about it on the Microsoft support page here: https://support.microsoft.com/en-us/office/focused-inbox-in-outlook-for-mac-380b3c37-d43e-449e-a9c8-8444c3c6dbeb
Feel free to ask the community if you have other questions and we fellow members will be glad to help!