So I've been trying to send email to any address @optonline.net. I submitted a request through their email abuse department but had no help whatsoever. This is a copy of the bounce back email:
Diagnostic-Code: smtp;550 5.7.1 Your mail from IP 184.108.40.206 was rejected. We can't currently accept your message. : firstname.lastname@example.org
Optonline support say our server is an open relay, it's not. I checked our server relay access and it's restricted to both our IP and localhost.
Does anyone know what could be causing this?