A person trying to send mail to someone outside of their organization gets the similar error of #553 Cannot find OUTBOUND MX Records for domain.
The e-mail domain you are trying to send to does not exist. For example, if you emailed someone at blahblahblahblah.com, it does not exist, therefore GreyMail tells you it does not exist and the message cannot be delivered.
<firstname.lastname@example.org> #553 Cannot find OUTBOUND MX Records for domain
Please check the email address you are trying to send to, as this is not a GreyMail operational problem.