Correction: large^W giant mail systems, that can observe a significant portion of world's mail, and have a lot of R&D resources, do a very good job at fighting spam (in their own systems).
A DIY home mailserver will still let, like, 5-15% of spam emails slip through.
The last few times I went to spam and saw emails that shouldn't be in there, a careful inspection revealed they were actually very well crafted phishing messages.
Ever since, I don't bother checking the spam folder. Too dangerous. Gmail is smarter than me.
A DIY home mailserver will still let, like, 5-15% of spam emails slip through.