Carlos E. R. wrote:
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The Monday 2006-11-27 at 12:30 +0100, Sandy Drobic wrote:
Just to clarify a few general things about SMTP:
I'm saving these notes ;-)
- the HELO string that the sending server is using
is not derived from DNS, it's usually the hostname
that is configured locally on the machine. RFC 2821 says,
this string has to be a resolvable FQDN for a domain/host.
Fortunately (for me) not many enforce this.
A lot of host do not have correct HELO implemented, Postfix can check this
with REJECT_UNKNOWN_HOSTNAME (for Postfix 2.3+ this has been renamed to
REJECT_UNKNOWN_HELO_HOSTNAME).
Even big Internet companies like Yahoo and Google have misconfigured HELO
for their servers, so it's not recommended to use these restrictions on a
general mailserver.
Postfix doesn't even have a check that says the a record and helo must be
identical, and for good reason. To number of false positives would be
astronomical. (^-^)
It is considered "best practise", but it's not what is implemented on many
servers.
- a sending server does not neccessarily need a MX record,
only correct A record and reverse DNS
Unless some one enforces it on the receiving end as an antispam measure, I
guess :-?
Then he's an idiot. Especially big companies that are sending millions of
mails per day have dedicated send-only mailservers, so it's NOT a good
idea to demand that the very same server that is sending a mail is
required to accept mail back for the sender address.
- the MX record is used to announce servers that will accept
mail for a domain, not neccessarily server that will send
for a domain.
- if the server is both sending and receiving mails for a domain,
then all records (MX, A, reverse DNS) are neccessary.
- if no MX record is set for a domain, mail will be sent to
the A record of that domain. Though you should set a MX record.
I think that it is usually rejected as domain must exist or similar
message, but again, that may be an antispam measure.
I encountered such a weird "antispam" measure only once. This is not
covered by any RFC. RFC 2821 explicitely says, if no mx record exists for
the recipient domain then the server has to resolve the a record of the
recipient domain and deliver the mail to that server.
Sandy
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