The sender-SMTP MUST ensure that the <domain> parameter in a HELO command is a valid principal host domain name for the client host. As a result, the receiver-SMTP will not have to perform MX resolution on this name in order to validate the HELO parameter.
The HELO receiver MAY verify that the HELO parameter really corresponds to the IP address of the sender. However, the receiver MUST NOT refuse to accept a message, even if the sender's HELO command fails verification.
Verifying the HELO parameter requires a domain name lookup and may therefore take considerable time. An alternative tool for tracking bogus mail sources is suggested below (see "DATA Command").
Note also that the HELO argument is still required to have valid <domain> syntax, since it will appear in a Received: line; otherwise, a 501 error is to be sent.
When HELO parameter validation fails, a suggested procedure is to insert a note about the unknown authenticity of the sender into the message header (e.g., in the "Received:" line).