False positives about disposable-email-domains HOT 21 CLOSED

Domains without an MX can still receive mails. The MTA will try the A record to deliver mails to this domain.

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The lookup first attempts to locate an MX record associated with the name. If a CNAME record is
found, the resulting name is processed as if it were the initial name. If a non-existent domain error is
returned, this situation MUST be reported as an error. If a temporary error is returned, the mess age
MUST be queued and retried later (see Section 4.5.4.1). If an empty list of MXs is returned, the
address is treated as if it was associated with an implicit MX RR, with a preference of 0, pointing to that
host. If MX records are present, but none of them are usable, or the implicit MX is unusable, this
situation MUST be reported as an error.

from https://tools.ietf.org/html/rfc5321#section-5

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with no MX record can the domain be used for mailing in any way?

Of course not.

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Have a look at https://github.com/ereli/disposable-email-domains/tree/validate
you can run python ./validate_dns.py |awk -F" - " '{print $2 }'|sort|uniq -c
and find out how many domains are no longer actively used. I should probably retry on NoAnswer, but these are the basic numbers I've got:

1355  MX found
49  no resolver NS servers found
384  non existent domain
391 NoAnswer

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One solution would be to use Travis CI:

1. Is it registered? - whois
2. Does it have an NS? - root servers
3. Does it have an MX? - DNS
4. Is the first A record on our parked domain list?
5. Is any of the MX-s on our MX blacklist?
6. Are all of the MX-s on our MX whitelist?

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These are great ideas. I am not that educated in domain system - with no MX record can the domain be used for mailing in any way?

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@szepeviktor So I guess good first step would be to move domains with no MX record out of the list. Something like discontinued_domains.conf.

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Mhmm, what about people who use this list to prune databases from disposable email addresses, both old and current? Most disposable email providers have a 45 days sunset policy and the domains are then dropped after that time. Should they not be listed anymore?

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@chafreaky That is a use case that would be covered by transferring the non-existent-MX domains to discontinued_domains.conf - would it not?

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This issue is more about some kind of quality control, keeping things up-to-date.

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@szepeviktor To be frank I think non-existent MX entry does not make (former) disposable domain trustworthy per se.

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The MTA will try the A record to deliver mails to this domain.

Could you link the reference of this?

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https://en.wikipedia.org/wiki/MX_record#History_of_fallback_to_address_record

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Thank you.

In January 1986, RFC 973 and RFC 974 deprecated the MD and MF records, replaced them with MX, and defined the MX lookup with fallback to A.

I've ran through the two RFC-s and found no traces of the mentioned fallback.

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Thank you!

I think it is not implemented in modern MTA-s.

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Sorry, but it seems you have no idea how mail works. It's been the standard for over 30 years. Every major MTA implements it as it is in the RFC as quoted above.

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Domains without an MX can still receive mails. The MTA will try the A record to deliver mails to this domain.

Thanks for the info @ScottyTM, this complicates things a bit further for this issue.

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@ScottyTM is right. If there is no MX, the A will be used as fallback.

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In a way this is a duplicate of #58.

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Yes.

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