Sender which is using postmaster email address is prohibited on Exabytes mail server. This feature is set with a line on our goal on spam fighting as we notice that there is a high number of spam which is referring to postmaster mailbox sender.
Due to this is the server wide settings, the domain level bypass is not available.
The sender need to consider on having the sender mail changed from postmaster to other mailbox address. If they are not sure on the settings need to be done, they may contact their mail server administrator for the setting change.
For further information, feel free to contact our support team at http://support.exabytes.com/help .