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?
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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