Diagnostic-Code: smtp; 541 5.4.1 Mail rejected by destination domain

When I send mail to your customer I now (past two days) get the following failure:

"A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

bryan@bryanbarkerbuilders.co.uk
   host mx.zoho.com [165.254.168.50]
   SMTP error from remote mail server after RCPT TO:< bryan@bryanbarkerbuilders.co.uk >:
   541 5.4.1 Mail rejected by destination domain
Reporting-MTA: dns; shcp04.hosting.zen.net.uk

Action: failed
Final-Recipient: rfc822; bryan@bryanbarkerbuilders.co.uk
Status: 5.0.0
Remote-MTA: dns; mx.zoho.com
Diagnostic-Code: smtp; 541 5.4.1 Mail rejected by destination domain"

Please advise your customer ( bryan@bryanbarkerbuilders.co.uk) when your servers will stop bouncing my e mails. Many thanks,

Robert Weir