A server sending mail to your domain is getting the messages rejected. The bounce back being sent includes the error message "554: Domains without a subdomain are not allowed."
554: Domains without a subdomain are not allowed.
This is caused when the sender system does not include a top-level domain in the 'mail from' line of the smtp transmission. This information is required by Symantec Brightmail Gateway antispam in order to process the message.
Inform the sender that the top-level domain data must be given when transmitting a SMTP message through Symantec Brightmail Gateway antispam.