2.RBL( Real-time black lists )
This technique, commonly referred to as RBL(real-time black-hole lists), checks the incoming IP address against various Black Lists to verify that the sending server is not listed as an open mail relay that spammers can use to relay their unsolicitied emails.
Normally any secure mail server should refuse to relay (send) email from an external sender to anybody outside its domain. This ensures that spammers cannot hijack it and use it to send spam that will look like it is coming from a legitimate mail server.
Unfortunately some system administrators, for whatever reason, fail to configure their mail servers to block such a relay.
eScan comes with a default set of the two most reliable free RBL services. eScan can be configured to use several RBLs at a time for higher accuracy.