Microsoft has over the years worked hard to enhance the junk / spam handling features in both the Outlook client and Exchange Server (not to mention Exchange's cloud "big brother" at outlook.office.com).
At one point, Microsoft had a feature called Clutter but they took that out (read more at https://www.howtogeek.com/407202/microsoft-are-getting-rid-of-clutter-in-outlook-and-thats-worrying/ ).
Recently however, I was trying to get an account reset e-mail due to a service I use having a breach. Out of an abundance of caution, the service reset all user passwords. Thus when I went to log in, I had to get a reset link.
The problem? I wasn't getting the e-mail. That's when my brain clicked into high gear (ok really after a few minutes).
A couple of weeks/months prior, I had started getting an occasional e-mail from Office 365 like the following one I received today:
Turns out I had a bunch of mail in there to review including my missing password reset e-mail.
The URL is: https://protection.office.com/quarantine .
If you log in as an Office 365 admin you'll see lots of stuff:
If you're a regular user there are usually a lot fewer options:
I hope this helps you find mail you may be missing.