I posted a topic in our Announcements category. A few groups are automatically watching the first post of that category, but every once in awhile I want everyone to receive something. This was one of those cases, so I mentioned @trust_level_0. I’ve done this before and it works fine.
Unusually, in this case, I had to reply to the original announcement. (This was the only reply.) I neither wanted, nor expected everyone to receive a notification about this post. I only wanted to add some information to the topic for those looking for it later.
However, 25 users received an email notification about my reply. It looked like this the Emails > Sent screen in the control panel:
See max users notified per group mention site setting.
Also, if people saw the post because they were online, they’ll get a notification on the browser, but not an email (unless they have turned on the email-me-harder setting in their own preferences).