On Wed, Jul 05, 2017 at 02:41:30PM -0500, /dev/rob0 wrote:
> o The domain name given in the EHLO command MUST be either a primary
> host name (a domain name that resolves to an address RR) or, if
> the host has no name, an address literal, as described in
> Section 4.1.3 and discussed further in the EHLO discussion of
> Section 4.1.4."
>
> That's a MUST. Either resolve to an address or use an address
> literal.
However: Section 4.1.4:
An SMTP server MAY verify that the domain name argument in the EHLO
command actually corresponds to the IP address of the client.
However, if the verification fails, the server MUST NOT refuse to
accept a message on that basis. Information captured in the
verification attempt is for logging and tracing purposes. Note that
this prohibition applies to the matching of the parameter to its IP
address only; see Section 7.9 for a more extensive discussion of
rejecting incoming connections or mail messages.
So we have is a requirement for the EHLO name, if not an address
literal, to be a real domain that resolves to some address (A or
AAAA). The server MUST NOT reject messages just because the name
does not correspond to the IP address of the connected client.
While there is no requirement to accept names that don't resolve
at all, in practice such a policy would block too much mail for
most sites, and is not recommended.
--
Viktor.
