Yahoo (particularly when providing services for BT) silently delete some spam, even mail with a raw SA score < 1.5.
And they don't respond to complaints, even from their own users.
I've been running the same pukka mailing list for 12 years, I'm in their abuse feedback loop, have proved exclusive ownership of the mail server, all mail is DKIM signed with valid SPF records, mail is accepted with a 250 OK, you name it.
Still they bin my emails, but only to some accounts. No rhyme or reason, no bounce, no spam folder. Just never arrives.
Greylisting does not throw anything out. It does the exact opposite. The mail server simply says, "I have a temporary problem, so I can't take this right now. Try again later" Mail servers are supposed to (and do) try again, at which time a greylisted message will be delivered. Accepting a message for delivery In Go Faith and then dropping it on the floor, is poor, lazy, cheap, RFC-breaking spam filtering.
What I mean is that greylisting is a trick mechanism where some mails, declared spam based on an unrelated and technical criteria, never reach the user's spam folder. This contradicts the 'spam should go to spam folder' point.