Linux Mail Blocked By Gmail 550

Question

I have a website set up on a CentOS server which has a contact form on it set up to send mail to a gmail address.

However the mail is being blocked by google with the following error:

Diagnostic-Code: smtp; 550-5.7.1 This message does not have authentication 
information or fails to pass
    550-5.7.1 authentication checks. To best protect our users from spam, the
    550-5.7.1 message has been blocked. Please visit
    550-5.7.1  https://support.google.com/mail/answer/81126#authentication for more
    550 5.7.1 information. q16si15869511wrs.198 - gsmtp

The mail can be received by a hotmail address, however it goes into spam.

I am not sure how I can authenticate the message, I have read this page: https://support.google.com/mail/answer/81126#authentication

Which suggests adding mx records, however there are already mx records for Gsuite emails on the domain.

Can anyone advise how to fix this issue?

Answer 1

You're likely sending messages with a "From:" address whose domain already requires either DKIM digital signatures to be present, and/or messages to be sent from only SPF-whitelisted servers.

Make sure the webserver sends messages only through the domain's mailservers and not directly. For example, if you're trying to use "From: […]@gmail.com", you must submit the message through smtp.gmail.com:587 (and using the corresponding Gmail account's login details).

Similarly, if you're using your own domain but it is managed by Google Apps (G Suite), then you've probably configured the SPF/DKIM/DMARC records according to Google's instructions – and that means you must now use the G Suite SMTP servers for all outgoing mail.

If you're using your own domain, but it is managed by a hosting company, then likewise you should use the outbound SMTP server provided by that hosting company. (These servers might be the same as inbound MX servers, but not necessarily.)

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If you're using your own domain and running your own mail system, then it could be that you've placed the mailserver on a network blacklisted by Gmail due to high spam rates. For example, hosting companies which use a shared IPv6 /64 pool for all customers are often blacklisted.

(In the specific case of Linode, requesting a dedicated /64 is the best option – that way Gmail will not mix your server's "reputation" together with others. Sometimes configuring the mail server to use only IPv4 will work around the problem as well.)

Answer 2

You need 2 things at least:

• Have a valid PTR record (reverse DNS for your server IP address)
• Have a valid SPF or DKIM record.

You can check both against an online dns validator tool

So basically do as Google says:

• The sending IP must have a PTR record (i.e. a reverse DNS of the
  sending IP) and match the IP obtained via the forward DNS resolution of the hostname specified in the PTR record.
• The sending domain should pass either a SPF check or DKIM check.