Reverse DNS is not a valid Hostname with existing PTR record matching HELO/EHLO

Question

There are a ton of questions related to this issue - and I have gone through almost each one yet this issue persists and I have no idea how to proceed further.

My mail server is setup with postfix, I have the corresponding A/AAAA records, etc setup including a PTR record from my IPv4 -> yourbud.co.za (my domain).

MXToolBox shows Reverse DNS does not match SMTP Banner. The reason being the HELO/EHLO response does not contain the domain.tld within the string. It does.

Requirements for SMTP server

• Postfix setup (dkim, dmarc, spf all setup too)
• A/AAAA records pointing yourbud.co.za -> ipv4/6 addresses
• PTR record pointing ipv4 (and ipv6 optional?) -> yourbud.co.za

Postfix (postconf)

someuser@yourbud:~$ postconf -d smtpd_banner
smtpd_banner = $myhostname ESMTP $mail_name

someuser@yourbud:~$ postconf -d myhostname
myhostname = yourbud.co.za

MXToolBox

You can find the MXToolBox MX diagnostics for my domain here. Further, dmarc, dkim & spf all report passes.

Root Problem (I need to solve)

Outlook and GMail are marking my emails as junk.

More Information:

According to this, and this, this, and this (and a few others) all suggest that having a banner mismatch will cause mail providers (Outlook, GMail, etc) to mark the emails as spam/junk which is what is happening.

You can find more information about my question & my setup here.My IP is not on any blacklist site.

According to this post, MXToolBox requires a subdomain handling mail, something like mail.yourbud.co.za but this isn't a requirement as far as I can tell.

Why does Outlook/GMail still regard my mail as spam/junk?

Answer 1

Your setup is probably fine

A matching PTR is not a requirement by any standard, although some mail systems will use it as a potential spam indicator increasing the score. However, from the details provided in your question it's easy to come up with an answer that the warning shown in MXToolBox is indeed a false positive, and not the root cause for your messaged being marked as a spam:

• Both IPv4 and IPv6 have matching forward and reverse records:

  yourbud.co.za. IN A 139.162.179.128
  128.179.162.139.in-addr.arpa. IN PTR yourbud.co.za.
  
  yourbud.co.za. IN AAAA 2a01:7e01::f03c:92ff:fed4:25b5
  5.b.5.2.4.d.e.f.f.f.2.9.c.3.0.f.0.0.0.0.0.0.0.0.1.0.e.7.1.0.a.2.ip6.arpa. IN PTR yourbud.co.za.
  

• SMTP banners on both IPv4 and IPv6 do match these records:

  Connection to 139.162.179.128 25 port [tcp/smtp] succeeded!
  220 yourbud.co.za ESMTP Postfix (Ubuntu)
  
  Connection to 2a01:7e01::f03c:92ff:fed4:25b5 25 port [tcp/smtp] succeeded!
  220 yourbud.co.za ESMTP Postfix (Ubuntu)
  

How DNS, DKIM, SPF and DMARC are set up plays just a minor role as a positive indicator regarding spam filters. Failing the tests of course causes the message treated as forged, but that's all: because spammers can set up these too, they alone can't be used as a solid proof of the quality of the contents.

It's the contents that matters

From the web page https://yourbud.co.za/welcome#HowItWorks I can see you are working on an industry that can be easily misunderstood, and it's well possible that your messages are being marked as junk simply because of their contents:

How it works

• Join South Africa's first Marijuana Co-Op Platform
• Search the best marijuana in your area
• Have it securely delivered to your door
• Sit back, relax and enjoy your personal use marijuana legally

Susan Gunelius (Cannabiz Media): The Trouble with Email Marketing for Marijuana Businesses:

The reality is that marijuana is still illegal at the federal level, and that means most email marketing application providers don’t like marijuana-related businesses. Email marketing application providers require that their users follow “acceptable terms of use” and those terms typically prohibit sending messages about illegal drugs, goods, or services – including marijuana.

Microsoft, Gmail etc., being US companies, fit into this conclusion, too.

Answer 2

MXToolBox shows Reverse DNS does not match SMTP Banner. The reason being the HELO/EHLO response does not contain the domain.tld within the string. It does.

Your MXToolBox output shows SMTP Banner Check: OK - Reverse DNS matches SMTP Banner, and you are correct that your EHLO responses use your hostname, which is valid. Rather, the warning given by MXToolBox is SMTP Valid Hostname: Reverse DNS is not a valid Hostname. Why it is saying this, I cannot fathom; your A, AAAA, and PTR records are all fine.

Regarding SMTP Banner Check, even if this didn't pass, it certainly wouldn't affect delivery of mail for your domain, since your server only issues an SMTP banner and EHLO response when other servers are talking to it to deliver incoming mail for your domain. When <alice@example.com> sends mail to <bob@yourbud.co.za>, the example.com SMTP server (let's say this is smtp.example.org) just looks up the MX record for yourbud.co.za and dutifully sends the mail there — it doesn't care about how your server responds to its message, EHLO smtp.example.org; if it did, then Alice would have reason to complain to their mail provider for failing to deliver mail they legitimately intended to send to Bob.

With regards to successfully delivering outgoing mail for your domain, the Postfix variable you actually need to worry about is not myhostname (which is used when another server talks to your server to deliver incoming mail for your domain), but smtp_helo_name (which is used when your server connects to another server to deliver outgoing mail from your domain). However, smtp_helo_name is set to $myhostname by default, so you probably already have this set correctly.

Since myhostname is set correctly, you shouldn't have any technical issues whatsoever with delivery of mail unless you have altered smtp_helo_name.