On Tue, Jun 16, 2009 at 6:20 PM, Franck Martin<franck(_at_)avonsys(_dot_)com>
wrote:
I recently encountered the following question/problems.
I have a mail server and one of my users complains he is not receiving
emails from a domain. How do I find if I have blocked the domain from
sending to my server. Meaning, knowing the domain name of the sender, how do
I find the IPs from where the mail could be sent from. It seems that SPF is
the only tool to provide that answer?
One approach that might help you is to go to senderscore.org (from
ReturnPath). Register for a free account and then enter in the domain
name. For example, when I enter in avonsys.com it shows me that there
is one IP address sending mail for that domain - 76.203.192.33 with a
hostname of adsl-76-203-192-33.dsl.rcsntx.sbcglobal.net.
Hope this helps.
In another related problem, which is linked to IPv6 and RBL. Buidling an
IPv6 RBL could lead to a huge database. Sure you can alleviate by using
"wildcards", but why not use the reverse DNS resolution to add a TXT record
associated to the IP to indicate the IP is the one of a mail server? So any
IP that does not have this record would be blocked for SMTP. As IPv6 is not
used for SMTP (or barely), this could be made mandatory for IPv6 and
optional for IPv4. An MUA could talk to an MTA on port 25 because we know
the the etwork range of the MUA or the alternative is to use the new mail
submit port.
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