Email Bounce back

[#1]

Hello,
One of my customers tries to send me emails and the bounce back as spam. blow is the NDR message
This is the mail system at host mx496501.smtp-engine.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
<xxxxxxxxxxxxxxxxxxxxx>: host mx.avasin.plus.net[212.159.8.200] said: 552
    Spam Message Rejected  (in reply to end of DATA command)
The client is definitely not a spammer
Please let me know what other info you might need.
Peter

[#2]

Hi Peter,
Ask them to send the email to a non Plusnet address you have and send me a private message containing the content of that email and the headers. I should be able to work out from there what the problem is.