550 Receiver does not accept unencrypted connections

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#1 Fri, 10/21/2016 - 17:04

550 Receiver does not accept unencrypted connections

I am running Virtualmin GPL with all the usual email stuff, Postfix, Milter, Greymin, Spamassassin and so on which runs out of the box in Virtualmin. Email domain has SSL. The system sends and receives 1,000s of emails each month without any problems.

Only from one of my clients I get this message whenever an email is sent to them (no matter if from our web shop or an ordinary mail) :

This is the mail system at host my.domain.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

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

                The mail system

user@my-clients-domain.com: host my-clients-domain.com[] said: 550 Receiver does not accept unencrypted connections (in reply to RCPT TO command)

Changed the email address, server names and IP in the copied message above of course, so that it is not exposed to the public, but this is what my Postfix says.

My system of course uses AUTH SMTP and it's only this mail server where I have problems with outbound mail. I receive emails from them without problems. There is not very much to be found about this error on Google and also not here on the forums. Would anyone have an idea how to solve this?