This article covers how to filter incoming emails by country. An email blocked by this filter will receive an error like the following.
550 Your country is not allowed to connect to this server.
1) Log in to WHM and navigate to the following location.
Home » Email » Filter Incoming Emails by Country
2 Use the toggle to the right of a country to switch it between allowed and blocked. Domains highlighted in green are allowed, and domains highlighted in red are blocked.