Resources Opinion: Is It Time to Replace SMTP?
Problem When you implement greylisting on your mailserver you can have a problem to send e-mails to other mail servers which implement Sender Address Verification (SAV). You will see logs such as: Mar 17 14:17:49 ns2 postfix/smtp: 45D49241802: to=<firstname.lastname@example.org>, relay=receiver.server.com[X.Y.Z.Q]:25,
SAV stands for Sender Address Verification. Before accepting an email a email server verifies if the sender exists. Because VRFY commands are disabled on many systems some systems veryfy the sender initiating a SMTP connection and then intercepting it. THIS