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 -- List replies only please! Please address PMs to: news-reply2 (@) japantest (.) homelinux (.) com -- To unsubscribe, e-mail: opensuse+unsubscribe@opensuse.org For additional commands, e-mail: opensuse+help@opensuse.org