Problem
Users report that some messages are returned with the explanation that the SMTP greeting
failed because no valid DNS name or pointer record was identified.
Discussion
When a message is sent through SMTP, the client and server are expected to identify each other.
This identification is part of the SMTP greeting.
The following example is a simplified version of a mail server (mail.sender.com) successfully transmitting a message to user@recipient.com:
- mail.sender.com connects to mail.recipient.com
- mail.recipient.com accepts the connection and identifies itself "Hello, I'm mail.recipient.com".
- mail.sender.com identifies itself "Hello, I'm mail.sender.com".
- mail.recipient.com acknowledges mail.sender.com and says, "OK, give me your message".
In the above example, mail.recipient.com has allowed mail.sender.com to transmit the message. In order to prevent spam, some
mail servers will validate the name provided by the connecting mail server before allowing the message to be transmitted.
The following example is similar to above, however mail.sender.com will provide a false name, and therefore mail.recipient.com will refuse to accept the message.
- mail.sender.com connects to mail.recipient.com
- mail.recipient.com accepts the connection and identifies itself "Hello, I'm mail.recipient.com".
- mail.sender.com identifies itself "Hello, I'm mail.anothersender.com".
- mail.recipient.com determines that mail.anothersender.com is someone else, or does not exist and says, "You are not who you say you are, Goodbye".
mail.sender.com will then return the email to the original sender address, quoting the reason why it could not deliver the message.
For example:
I'm sorry but I could not deliver your message.
The remote mailserver (mail.recipient.com) says:
"You are not who you say you are, Goodbye"
Solution
Set the Internet hostname (e.g. mail.domain.com) of the mail server in Configuration -> Domains -> 'Internet hostname'.
This domain name must resolve to the Internet IP address used by Kerio MailServer for Internet communication.
