Cannot send to some mail servers with the explanation that the SMTP greeting failed

Issue encountered

Users report that some messages are returned with the explanation that the SMTPSimple Mail Transport Protocol - An internet standard used for email transmission across IP networks. greeting failed because no valid DNSDomain Name System - Enables the translation of hostnames to IP addresses and provides other domain related information. name or pointer record was identified.

Causes

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:

1. mail.sender.com connects to mail.recipient.com
2. mail.recipient.com accepts the connection and identifies itself "Hello, I'm mail.recipient.com".
3. mail.sender.com identifies itself "Hello, I'm mail.sender.com".
4. 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.

1. mail.sender.com connects to mail.recipient.com
2. mail.recipient.com accepts the connection and identifies itself "Hello, I'm mail.recipient.com".
3. mail.sender.com identifies itself "Hello, I'm mail.anothersender.com".
4. 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 addressAn identifier assigned to devices connected to a TCP/IP network. used by Kerio MailServer for Internet communication.