I posted a reply to this thread a few years ago wondering the same thing as you. I discovered that, ultimately, it means that your email address was included in a data breach for something not related to outlook or Microsoft. As mine was clearly an outlook email address, the attempts to login to my outlook were incessant using the password that was included in the data breach, like 20-30 a day. I disabled the ability for the email address to be used to login to my outlook, (I'm now using a different one that I use for nothing else) and the attempts to access my outlook stopped immediately.
Separately, every now and again, I get requests for one time codes and these relate to websites where you used your outlook email address to register. Unfortunately, Microsoft does not give you the ability to track where your outlook email is being used to login. It's a deficiency that has been pointed out time and again for years and they never fix it.
My recommendations:
- Go through your inbox and search for emails with "subscription" or other related terms and identify every website you have subscribed to using the affected email address.
- Go to "haveibeenpwned" (google search) and input your email address. This website has indexed all the data breaches the contents of which have been posted on forums or the dark web. It's quite useful. This should help identify which data breaches your email address was included in.
- Go to the websites you have identified and either request that your account be deleted or change how you login to it. This should stop most of the one time codes
Hope this helps, I know the so called Microsoft experts are completely useless and clueless.