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Re: [spfbis] RFC7208 4.6.4 Interpretation - MX Lookup Count Inconsistencies

Klaus Frank <klaus.frank@posteo.de> Thu, 02 February 2023 10:45 UTC

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Subject: Re: [spfbis] RFC7208 4.6.4 Interpretation - MX Lookup Count Inconsistencies
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The 10 lockups is a real issue that I encountered before.

Example for where the 10 lookups can be an issue, when a company uses a 
hand full of popular services:
* Zendesk: include:mail.zendesk.com (1)
* Mailjet: include:spf.mailjet.com (1)
* Zoho: include:one.zoho.eu (4)
* AWS Simple Email Service: include:amazonses.com (1)
* Gmail/Google Workspace's: include:_spf.google.com (4)

This selection isn't unreasonable and already yields a total of 11/10 
lookups...

On 23.01.2023 05:05, Jan Schaumann wrote:
> Tim Wicinski <tjw.ietf@gmail.com> wrote:
>
>> I also feel that technology stacks have matured over time.  Jan, do you see
>> real world examples
>> of a domain with 11 MX servers?
> I just happened to be looking at the MX records for
> the top 1M domains for other reasons, and counting
> domains that have > 10 MX records, I find at least 265
> domains.
>
> The winning price goes to the apparently rather aptly
> named domain 'everymailbox.com', with 398 MX records.
> Second prize goes to 'preciseify.com' (266 MX
> records), 3rd to 'rm02.net' (235 MX records). :-)
>
> Those are obviously rare outliers, of course.
>
> But I'm actually less concerned about domains with >
> 10 MX records (since the RFC seems reasonably clear
> here to immediately fail), and more worried about
> domains that have close to 10 total lookups, but where
> counting the lookups resulting from turning MX results
> into IP addresses would bump them over the limit of
> 10.
>
> As illustrated by Mark's initial mail (and my own
> misunderstanding), there are implementations that will
> lead to such domains' SPF records as being marked
> invalid when they shouldn't be.
>
> (The last time I looked at SPF records of popular
> domains[1], I found >8K domains with >10 total
> lookups.)
>
> -Jan
>
> [1] https://www.netmeister.org/blog/spf.html
>
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