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domain name system - Mailgun mails bouncing, possible DNS records are wrong?

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This has probably been
asked and answered before, but I'm a bit lost because I don't know what's happening and
therefor don't know what to look for. I would not only like a solution of course, but I
would also like to understand what's happening. I have a technical background, but in
software development. Servers, DNS records, etc is a bit new to me (although I've
managed).



I'm running a web application on
shared hosting. I have access to a Plesk control panel (I believe 12.5). The domain name
is registered at another company. And for sending mails, I'm using Mailgun (calling
their API).



Now, some mails bounce (others
don't), with messages
like:




  • Sender address
    rejected: Domain not found

  • sorry, your domain does not
    exists.




When I
use MXToolbox, an href="https://mxtoolbox.com/SuperTool.aspx?action=mx%3apeergroups.be&run=toolpage"
rel="nofollow noreferrer">MX Lookup looks fine. But when I test the email
server (with MXToolbox), I see the following
messages:




  • Reverse DNS does
    not match SMTP Banner

  • Warning - Does not support
    TLS



I don't think the
second is a problem (?), but the first one might be? When I do a check for SPF I
see:





  • DNS Record
    not found



Should I fix
this? (I've heard about SPF in mail contexts) href="https://serverfault.com/questions/591046/why-does-the-mxtoolbox-spf-record-checker-not-like-my-spf-record">This
question seems to suggest I should add an SPF record, but I currently can't in
Plesk 12.5.



On the other hand, Mailgun itself
says everything is fine (apart from the records but as I read it, they're not important
for sending mails?):
rel="nofollow noreferrer"> alt="Mailgun DNS">



So what's going
on here? And how can I fix it, or what should I know to dig
deeper?




Update



The
domain this is happening on is peergroups.be



An
example of headers of a mail that was
received:




Delivered-To:
peter.morlion@gmail.com
Received: by 10.100.161.143 with SMTP id
q15csp289063pjc;

Wed, 24 May 2017 04:10:04 -0700
(PDT)
X-Received: by 10.98.93.217 with SMTP id
n86mr37944933pfj.113.1495624203335;
Wed, 24 May 2017 04:10:03 -0700
(PDT)
Authentication-Results: mx.google.com;
spf=pass (google.com:
domain of bounce+3c1c7e.55be3c-no-reply=peergroups.be@mg.peergroups.be designates
198.61.254.51 as permitted sender)
smtp.mailfrom=bounce+3c1c7e.55be3c-no-reply=peergroups.be@mg.peergroups.be;

dkim=pass header.i=@mg.peergroups.be
Received-SPF: pass (google.com: domain of
bounce+3c1c7e.55be3c-no-reply=peergroups.be@mg.peergroups.be designates 198.61.254.51 as
permitted sender) client-ip=198.61.254.51;
Received: by 10.99.134.67 with POP3
id x64mf19905540pgd.0;
Wed, 24 May 2017 04:10:02 -0700
(PDT)
X-Gmail-Fetch-Info: no-reply@peergroups.be 4 peergroups.be 110
no-reply@peergroups.be

DomainKey-Status: good
Received:
from so254-51.mailgun.net ([198.61.254.51]) by home with MailEnable ESMTP; Wed, 24 May
2017 06:15:04 -0400
DKIM-Signature: a=rsa-sha256; v=1; c=relaxed/relaxed;
d=mg.peergroups.be; q=dns/txt; s=pic; t=1495620904; h=Content-Type: Mime-Version:
Subject: From: To: Message-Id: Date: Sender;
bh=60eeErLSy3DUfbYC4arXt0KAzdkuCC532GBme+wUcGk=;
b=ky0zW94QsXQqkl8LFf+S0YI87ltc92JRKbl4sxN2HKe6ZJwsBBfIJMr5IQGg+LVBCxb0wt3b
jcAVWRGFzHVXmIk/y5Ejphp1LwrkKKg62rocD6Jx4ZZFLsDiTMaXa3k108wnEhQuK4vbiEZP
QUtpRzdcoaYC5AtFzoaQ9PYPU6g=
DomainKey-Signature: a=rsa-sha1; c=nofws;
d=mg.peergroups.be; s=pic; q=dns; h=Sender: Date: Message-Id: To: From: Subject:
Mime-Version: Content-Type;
b=O0C2WmpWCejy1cmQF1zc8spQo3zdEe7EV14Niynve2ilpUBguylHYU/muTCkD+n6saCKjK
fvyjkBsvkxI6r2dA1myRBq8RQ9jZjgFxPjG6QgVSw9xObYz69QssRGrEAIZonOUMjoTzqrbd
uDAM11KpxXB488y9NzH8LmzaXTAig=
Sender:
no-reply=peergroups.be@mg.peergroups.be
Date: Wed, 24 May 2017 10:15:04
+0000
X-Mailgun-Sending-Ip: 198.61.254.51
X-Mailgun-Sid:
WyIxMjkwYSIsICJuby1yZXBseUBwZWVyZ3JvdXBzLmJlIiwgIjU1YmUzYyJd
Received: by
luna.mailgun.net with HTTP; Wed, 24 May 2017 10:15:03 +0000
Message-Id:
<20170524101503.72580.5AE8265694D87481@mg.peergroups.be>

To:
no-reply@peergroups.be
From: Peergroups

Subject: Some subject
here
Mime-Version: 1.0
Content-Type: multipart/alternative;
boundary="c9281a23246040cfb8a7081ab45d40f5"
Return-Path:



Answer




It seems I made a small mistake where I added the MX records for mailgun using
the peergroups.be domain, but Mailgun was using
mg.peergroups.be Adding MX records for
mg.peergroups.be solved it. This also became apparent in the
Mailgun dashboard, where the orange warning signs you see in the screenshot above became
green checkmarks.


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