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g suite - Configuring SPF, DKIM, Reverse DNS for VPS sending email through Google Apps SMTP servers

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Today we realized that all of our
emails being sent to email addresses of the type
@corporatedomain.com were getting rejected. I
don't know who is hosting the email for corporatedomain.com. I
don't think that should matter. Anyway, the error we were getting was



Technical details of permanent
            failure:
Google tried to deliver your message, but it was rejected by the
            server for the recipient domain example.com by mail.example.com.
            [yyy.yyy.yyy.230].

The error that the other server returned
            was:
550 5.7.1 Command
            rejected




  • We
    have a Google Apps account for mydomain.com. The DNS for
    mydomain.com had the SPF record v=spf1 mx
    include:spf.mailjet.com -all    .

  • We are using
    the Google Apps SMTP server to send transactional emails from with my app. We am using
    the mailjet account to send bulk marketing emails.

  • Since
    the mail for this domain is handled by Google Apps, the MX records all point to the
    default Google app ones.

  • My VPS has two public ip
    addresses. I don't have an SMTP server installed on my VPS. I send emails from within my
    VPS using the Google Apps SMTP server ONLY.

  • Only the
    emails sent via the Google App SMTP server were getting blocked. The Mailjet ones
    delivered fine.



Given
the above usage pattern & VPS environment setup, I assumed the SPF would checkout
properly (the MX records point to Google and I am using their SMTP servers to send
email).




Googling the error really
didn't lead to anything. Eventually I tried carpet bombing the issue by including
whatever I could think of in the SPF. And now it works. Cool! The spf entry now
reads



v=spf1
            include:_spf.google.com include:spf.mailjet.com ip4:xxx.xxx.xxx.143 ip4:xxx.xxx.xxx.144
            mx ~all


But I am not
sure WHY it works. I really DONT want to do a trial & error
to find the bare minimum settings that work for the recipients domain. There must be a
spec for these things, right? After all, all emails were/are getting delivered to all
users (not with emails belonging to
corporatedomain.com).



I
have a suspicion that in the current SPF entry, the ip4
mechanisms as well as the mx one are not needed. Google
documentation recommends using include:_spf.google.com instead
of mx. Also, this one ends with less restrictive
~all. The old SPF ended with the highly restrictive
-all. Can someone confirm what the ideal SPF record should be
for my use case?




Side note: I have
now configured DKIM authentication for mydomain.com. I don't
know if I need to setup Reverse Dns. I don't think I do, as I am not "sending" the
email, the Google Apps SMTP server is. Please correct me if I am wrong.


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Answer




The
include:_spf.google.com you eventually added is likely the
critical bit here. It flags Google's servers as being legitimate senders of mail on your
behalf.


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