Email bounced - 550 5.7.54 SMTP

I have proxied DNS and strict full SSL but my emails have bounced.

This is the mail system at host relay.

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

[email protected]: host mail.hartman.com.au[203.176.184.191] said:
550 5.7.54 SMTP; Unable to relay recipient in non-accepted domain (in reply
to RCPT TO command)

That doesn’t sound like a Proxy issue, as mail doesn’t go through Cloudflare.

Are you able to send/receive email with other people?

Yes, I have been able to send to others,

Could it be the recipient’s antivirus settings ?

It looks to be more severe than just a user setting at the other end. You’ll need to contact your email host provider to assist with this.