All Email Security Gateways, firmware versions 3.5.12 and above.
When the Email Security Gateway receives a connection, it does a reverse DNS lookup using the IP address of the sender to find the hostname associated with it. The last part of a hostname is known as the top-level domain, or TLD. Most TLDs outside of the United States include a country identifier, such as .ca for Canada, .ru for Russia, and so on. If most or all of the mail that you receive from a given country is spam, you may choose to tag, block or quarantine any message that has an associated hostname that includes that country's TLD. Email which is not blocked is subject to all of the usual spam and virus checks.
To block, quarantine or tag messages from one or more specific countries: