Mail server op here: mail exchangers (mine included) absolutely silently drop insane amounts of email submissions without any indication to the sender or the envelope recipient. At most there's a log the operator can look at somewhere that notes the rejection and whatever rules it was based on.
I run a family email server. I’ve worked with a few mail sysadmins trying to diagnose why some of our mail doesn’t get through. Even with a devoted and cooperative admin on the other side, I can absolutely believe that general logging levels are low and lots of mail is silently discarded as we’ve seen that in targeted tests when we both wanted to see the mail and associated log trail.
The spam folder (or the inbox, in what you consider outlook) isn't for messages known to be spam; it's for messages that the filter thinks might have a chance of not being spam.
Most spam is so low-effort that the spam rules route it directly to /dev/null. I want to say the numbers are like 90% of spam doesn't even make it past that point, but I'm mostly culling this from recollections of various threads where email admins talk about spam filtering.
I run "gray listing" software on my email servers. Some spam is blocked by that. There are logs, but I never look at them, other than when I do an upgrade. Mail that makes it through will hit a secondary spam filter which will flag it as "spam" and dump it into a different folder (which I look at slightly more frequently.)
Tell me you've never run a mail server without telling me you've never run a mail server.
It's pretty standard practice for there to be a gradient of anti-spam enforcement. The messages the scoring engine thinks are certainly spam don't reach end users. If the scoring engine thinks it's not spam, it gets through. The middle range is what ends up in spam folders.
You need to read through wikipedia's article on anti-spam techniques! There are whole categories of techniques that require no intervention from humans.
This would forbid things like spam filters.