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Moriah
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PostPosted: Fri Jun 29, 2018 4:45 pm    Post subject: HELP - reverse dns and email pbm - SOLVED Reply with quote

I recently switched network connectivity providers, with a consequence that my ip address changed. I had my DNS entries changed for my web server and my email server, but the reverse DNS was not changed; therefore, my domain name resolves to the correct ip address, but that ip address does not resolve back to my domain name, it resolves to the cable privider's name.

I am having trouble with my sendmail mail server in that many people are unable to receive my emails. I can receive theirs ok. I can receive email that I send to myself, but that stays on my local network. My sendmail configuration has not changed since everything worked on the old ip address.

Is the reverse DNS likely the problem with others being unable to receive my emails?
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Last edited by Moriah on Sun Jul 29, 2018 11:50 am; edited 1 time in total
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szatox
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PostPosted: Fri Jun 29, 2018 6:57 pm    Post subject: Reply with quote

It is a very common check and many email servers do indeed reject emails from "wrong" domains, where MTA's advertised name does not match PTR.
Also, use DKIM. Telling the receiving server you're not a random bot is a good start, even if you never had a chance to build your reputation.
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bunder
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PostPosted: Fri Jun 29, 2018 8:45 pm    Post subject: Reply with quote

Yes, your A has to have a PTR and they have to match.
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Ant P.
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PostPosted: Sat Jun 30, 2018 3:46 am    Post subject: Reply with quote

Do you have a SPF record for the mail server? Adding that might help even if the PTR can't be fixed.
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Moriah
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PostPosted: Sun Jul 01, 2018 2:08 pm    Post subject: Reply with quote

@Ant P.
I am not running the DNS, and I am pretty ignorant about the email specific DNS records. Does the SPF record go in the name server, or the mail server? If it is something I can fix by editing my /etc/sendmail.mc file, that would make it easy.
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szatox
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PostPosted: Sun Jul 01, 2018 5:28 pm    Post subject: Reply with quote

SPF is a TXT record for bare domain.
DKIM is also a TXT record in _<selector>._domainkey.your.domain.

You can't set them if you don't control DNS
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Ant P.
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PostPosted: Sun Jul 01, 2018 8:08 pm    Post subject: Reply with quote

A simple SPF record would look like this (in `dig`):
Code:
;; QUESTION SECTION:
;example.org.                  IN      TXT

;; ANSWER SECTION:
example.org.           600     IN      TXT     "v=spf1 mx -all"

That's all that's necessary; it tells receiving hosts they should accept mail from your domain if sent from the IP address your MX record points to, but reject any others.

DKIM is a bit harder to set up and not always worth it, because it involves installing/configuring extra software. Safe to ignore it for now.
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Moriah
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PostPosted: Sun Jul 01, 2018 9:38 pm    Post subject: Reply with quote

I will speak with the DNS administrator tomorrow. Thanks! :D
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Moriah
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PostPosted: Sun Jul 29, 2018 11:50 am    Post subject: Reply with quote

The problem has been solved.
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