Blocked Top Level Domains
Go to System Setup > Mail Relay > Sender Controls > Blocked Top Level Domains (TLDs) to add, edit and delete domains that are rejected during the initial SMTP connection. For example, you could use this section to block all emails that come from .info domains.
To view a list of current top 10 malicious top-level domains (TLD's), visit https://www.spamhaus.org/statistics/tlds/ 
Adding a Blocked TLD
Click Add... and the Add TLD window displays.
In the Top Level Domain: field, enter the top-level domain you want to block. The TLD must be preceded with a '.', e.g. enter .biz to block all emails with a sender address ending with .biz.
Enter an optional comment for this TLD in the Comment: field.
Click Save.
Editing a Blocked TLD
Click the edit
icon in the Options column to the right of the TLD you want to edit.The Edit TLD window displays where you can make your changes.
Click Save to save your changes or Cancel to discard changes.
Deleting a Blocked TLD
Click the checkbox
to the left of the TLD(s) you want to delete and click Delete. To delete a single TLD, click the delete 
icon to the right under the Options column.