Re: [Asrg] What are the IPs that sends mail for a domain?

Douglas Otis <> Wed, 17 June 2009 00:24 UTC

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Subject: Re: [Asrg] What are the IPs that sends mail for a domain?
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On Jun 16, 2009, at 3:55 PM, John Levine wrote:

>> 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?
> Unless you have previous mail from the domain, I would agree SPF is  
> your best bet.

This is not your only bet.  Many SPF records include the term MX and,  
when not found, even default to using MX/24.

>> 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.
> We've had a variety of proposals to identify mail client hosts.  See

The CSV effort proved most providers do not want their MTAs identified  
as belonging to them, even when it could improve email acceptance.   
This might be especially true now after their support staff has been  

Reverse DNS is already causing a large amount of resources to be  
wasted by the shabby state of the reverse name space.  Incorrectly  
configured RFC 2317 delegation, and many non-functional servers are  
causing MTAs to rapidly become resource limited when making reverse  
checks.   In addition, when your customers conduct business with Asia,  
they may not be happy to find email is being lost as a result of  
geographic differences of opinion about the role that reverse DNS  
might play with email.

IMHO, all outbound MTAs should be required to return CVS records for  
their EHLO name and offer MX records for their inbound.  A mandate  
that required MX (inbound) or CVS (outbound) records would greatly  
help in identifying non-abusive email sources against a backdrop of  
hundreds of millions of bot-net controlled drones spewing email.   
Systems may soon use ACLs as a means to white-list safe MTAs.  Perhaps  
the world is a few years from having to go to that extreme.