What is meant by '550 Relay not Permitted' when it comes to emails?

Craig asked:

For the past few months, I have been emailing a company with no problems. They have been able to receive my emails and they have been responding.

A few days ago, they dropped me an email where my response was met with the following System Response:

This is the mail system at host mail.example.com.

I’m sorry to have to inform you that your message could not be
delivered to one or more recipients. It’s attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can delete your
own text from the attached returned message.

The mail system

: host
mx10.mailspamprotection.com[xxx.xxx.xxx.xx] said: 550 relay not permitted!
(in reply to RCPT TO command)

mail.example.com being my own Mail Server.

[email protected] being the email address I am trying to send mail to.

What exactly does this mean and is there anything I can do about this?

I have tried sending the email from both Outlook and directly within the Server, where I am met with the same problem.

According to MX Toolbox, I am not listed in any Blacklists nor does there seem to be any problems with my SPF and DMARC Settings etc. I also do not seem to have any problems sending or receiving emails with anyone else.

My communication is limited to emails, with this company, where I also sent them an email via their Contact Form on their website. I have yet to receive a response, via their Contact Form neither; so I am not sure whether that was received or not.

My answer:

This message “Relay access denied” or “Relay not permitted” means that the mail server received a message for a recipient at a domain that it doesn’t handle email for, and it is refusing to accept it on that basis.

While mail servers can be configured to accept and relay mail for domains they don’t serve, this is called an open relay and is very commonly abused by spammers whenever they find one.

In the instant case, the problem is not your mail server, but the recipient’s mail server. The mail is apparently going to a spam protection service which, for whatever reason, has decided not to handle mail for the recipient’s domain anymore. Maybe they didn’t pay their bill. Who knows. But it isn’t your problem.

View the full question and any other answers on Server Fault.

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