Re: Reverse Lookup & Spam

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From: Rich Matheisen [MVP] (richnews_at_rmcons.com.NOSPAM.COM)
Date: 12/09/04 

Date: Thu, 09 Dec 2004 10:37:58 -0500

"Lanwench [MVP - Exchange]"
<lanwench@heybuddy.donotsendme.unsolicitedmail.atyahoo.com> wrote:

>cldpeak wrote:
>> Is it common for various corporations to use reverse lookup (ptr
>> records) to verify e-mail and drop those that don't resolve as spam?
>
>It's starting to be more common, asinine though I find it.

Using the absence of a PTR record as an absolute yes-or-no decision
certainly is kinda weird. If it's just one of several conditions that
are checked then it's an okay test, but only if it's not heavily
weighted. On a scale of 0 - 100 it should probably rank somewhere
around a 1 or 2.

Using forward lookups is much more sensible. If there's no MX, A, or
CNAME for the domain it's probably forged. If it's not forged it's
probably not very well run and dropping the mail probably wouldn't
hurt. If it does, that's what white lists are for.

                                        [ snip ]

>> However, I remember a time when a client could not send mail (to GAP
>> inc. in particular) turned out they use reverse lookups and would not
>> accept mail until I had the ISP create the PTR records.
>
>The ISP, not the DNS host. Unrelated....

Correct. The owner of the network address block has to create the PTR
records, not the owner of the domain.

>> So, if running my own DNS I would not have the authority to create
>> the PTR record for an IP address that belonged to an ISP right? I'll
>> still have to pay the DNS hosting records with the ISP unless there
>> is a way around it...
>
>Not really relevant, because as I said you wouldn't create the reverse
>lookup for your public IP anyway - whoever owns that netblock would do it.
>>
>> opinions?

Sure. Any ISP that charges for adding the necessary PTR records is
probably one you want to avoid using. 

-- 
Rich Matheisen
MCSE+I, Exchange MVP
MS Exchange FAQ at http://www.swinc.com/resource/exch_faq.htm

Relevant Pages

  • Re: Not receiving e-mails
    ... I have contacted our ISP to check the MX record for me, ... We are not using reverse lookup so there is no PTR record in DNS. ... >> RobL ...
    (microsoft.public.exchange.misc)
  • RE: Mail error: reason: 550 5.0.0 You must use a valid mail serve r
    ... Why do I not run into the same reverse lookup problem? ... Verizon and got my PTR record changed. ... > will return my domain-name is for my isp to put a PTR record in their ... Delegate the PTR record lookup at your DNS server. ...
    (RedHat)
  • Re: Reverse Lookup & Spam
    ... > Is it common for various corporations to use reverse lookup (ptr ... > with the old ISP maintained DNS records. ... > accept mail until I had the ISP create the PTR records. ...
    (microsoft.public.exchange.admin)
  • Re: Reverse DNS for multiple mail domains on same server
    ... The PTR must uniquely resolve ... An ISP sets up email for 1000 client customer with different ... DNS zone/domain name but that ISP SMTP server can only have ONE ... there can be only one PTR record for a host ...
    (microsoft.public.exchange2000.admin)
  • Re: Reverse DNS for multiple mail domains on same server
    ... The PTR must uniquely resolve ... An ISP sets up email for 1000 client customer with different ... DNS zone/domain name but that ISP SMTP server can only have ONE ... there can be only one PTR record for a host ...
    (microsoft.public.win2000.dns)