Gmail will every now and then let absolutely ridiculous emails through the spam filter.
Badly spelled, bogus URLs, with $$$$ all over the place and All CAPS and just so clearly to any human a phishing attempt or just general scumminess and gmail takes a look at it and says “yup everything seems legit here. On to the inbox!”
Google has to intentionally inbox a tiny fraction of spam, otherwise they'll never get corrective feedback on false positives, because basically nobody looks in their spam folder. Also, they have to permit a teensy fraction of stuff that would normally be temp-failed at SMTP time, for the same reason: otherwise it would be impossible for address and network reputation to improve. So sometimes if you see something completely ridiculous get inboxed, it might be that they are just trying to get data.
Ah, that explains some of what I see. I had noticed that my GMail spam folder went from thousands to dozens. I assumed they were zapping most of it but wasn't sure.
The overwhelming majority of suspected spam is rejected with temporary failure codes in SMTP and never makes it to the spam folder. The stuff in your spam folder is the cream of the crop!
I've got the opposite problem - I get almost no spam to my inbox but regularly get legitimate emails routed to my spam folder, so I have to treat my spam folder like a second inbox anyway.
There is a whole economy around spam at some scales: sometimes the spammer (the one actually sending the mail) has convinced a scammer (or slightly less illegitimate seller) to pay them to send mail because they claim to be good at getting through - in those cases the spammer actually gets paid a small amount (depending on their situation, a small amount to us could be a huge amount to them) no matter what.
sending a mailshot through a botnet has zero marginal cost. Maintaining the botnet has overhead, but that's not a cost of sending the email, it's an overhead.
I doubt this. Because even bot nodes will get blocked eventually so each email is degrading that value of that node. Again, very, very low, but not exactly zero.
