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The situation:

  1. Even though SPF, DKIM, and the entire mail formatting is optimized to score 10/10 on mail-tester, many outgoing mails are marked as spam in yahoo and outlook.
  2. The primary server sends out the emails.
  3. Incoming email is handled with mailgun.org, so the MX records (on the primary server) point to mailgun servers.
  4. It is relatively easy to set up the PTR record for the primary server, but AFAIK impossible on mailgun.

Questions

  1. Is setting the PTR record on the primary server enough?

  2. If not: what is the exact way a major email service provider checks incoming emails regarding the PTR record?

    a) domain -> read MX record -> look up PTR of domain in the MX record (which would be mailgun)

    b) domain -> look up PTR record of that domain (primary server in this case)

    c) other?

Swiss Mister
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  • This is precisely *not* a duplicate. The question is what to do when incoming and outgoing email is handled by two different servers. – Swiss Mister Jun 15 '15 at 21:37
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    Why do you think that matters, either to whether the question is a duplicate or whether the recipients mark your outgoing mail as spam? – Jenny D Jun 16 '15 at 03:51

1 Answers1

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Generally speaking the receiving server will look at the client that has connected to it and asked to deliver some mail. The first part of the SMTP protocol is a greeting command (HELO or EHLO) where the client can identify itself. Most email servers will take that claimed identity and compare it to the PTR record for the client's IP address. The client could identify itself as anything it wants, which is why the PTR check is a good way to see if this client is trustworthy.

So the most important thing for you is to have a PTR on your main server that is sending your email (and double check what your server identifies itself as when sending!). Incoming mail doesn't matter because that's not the SOURCE of your email which is the only thing receivers care about.

It is possible that layers of anti-spam technology beyond the SMTP protocol (including proprietary ones at companies like Google) do some more involved checks against your domain and its DNS. For this, it should be enough to ensure that the main IP address(es) that point to your server have a sensible PTR set on each.

user109322
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