How to stop notifications from Office 365 Mail to my iwatch

Okay, you did not indicate that you were referring to the app, and not just a mail account. If you are receiving the notifications, and you still have the account on your phone, it is the phone notification that you are getting on the watch. Go to the watch and check Outlook for notifications. Not the "Installed on the Watch". Go to Notifications, then the list of apps there, and you will set how notifications are handled on the watch. If you do not want notifications to mirror your phone, then your watch would provide you the notification for that app. If you turn off notification for Outlook on the watch in the Notifications area, then you would just get the notification on the iPhone.

Notification settings for the Apple Watch can be controlled by the Apple Watch app on the phone.

You're welcome. I have several mail accounts in the native Mail app on my iPhone, but I only want one of them to give me a notification on my watch, and that is my work Exchange account, which is pushed. The other accounts are not that important, and I can check them periodically, so I have them turned off for watch notifications. Works perfectly.

Go to notification setting and turn it off.