RFC1123Requirements for Internet Hosts -- Application and Support
Status of This Memo
This RFC is an official specification for the Internet community. It
incorporates by reference, amends, corrects, and supplements the
primary protocol standards documents relating to hosts.
5.2.5 HELO Command: RFC-821 Section 3.5
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.
Примерный перевод последнего абзаца:
Получатель МОЖЕТ проверять соответствие параметра HELO
IP-адресу отправителя. Но получатель НЕ ДОЛЖЕН отказываться
принимать письмо, даже если параметр HELO не соответствует
IP-адресу отправителя.