Mentioning PayPal. This week I received a genuine mail from PayPal, the first for years, because I don't use them regurarly. Gmail put it into the spam folder, obviously the word PayPal is nearly equivalent to spam.
Generally Gmail spam folder receives several false positives each week, maybe some 5% of all detected spam.
Even more worrying is that both gmail and IBM hosted mailservice reject legitimitate mail before SMTP is complete. (I have seen this in logs often enogh, although I don't follow them systematically)
Obviously they have overlooked in their spam filters that mail can be forwarded automatically. So if a Paypal message is forwarded and arrives from a non-paypal server, it must be spam. If your forwarding server forwards a couple of spam messages, subsequent legitimite message might be rejected, because this server sends too much spam.
Yes, spam is down from 20 a day to maybe 5 a day for me. But how much much is caused by rejection during SMTP, I really don't know.