The Country Filtering feature allows you to configure restrictions based on the country name of the connecting peer IP, for the SMTP Incoming service.
In order to identify the country for a given IP, you can upload an IP to a country mapping database (in CSV format). The database can be downloaded from sources like MaxMind. A free version is available at GeoIPCountryCSV-lite
You can also configure the list of IPs or IP ranges to be skipped by the GeoIP look-up.