since a large percentage of spam has forged and invalid sender ("from") addresses, some spam can be detected by checking that this "from" address is valid. A mail server can try to verify the sender address by making an SMTP connection back to the mail exchanger for the address, as if it was creating a bounce, but stopping just before any e-mail is sent.
Callback verification is compliant with SMTP RFCs, but it has various drawbacks, mainly that it relies on external mail servers whose responses are not reliable, as well as the fact that it's using other people's resources to combat spam. At the same time, there will be numerous false negatives (spammers abusing real addresses) and some false positives (legitimate e-mail with undeliverable sender address).
