Here is what I use to block all foreign or country specific domains.
From --- Matches Regex
.*@.*\.(?!(.*\.)?(com|net|org|int|edu|gov|biz|mil|coop|us))
This will only let through emails coming from .com, .net, .org, etc.
The downside is that the web is changing, there are now .ws and other domains that are legit and not top level country domains. I've had to tweak this filter a lot by combining it with other filters using AND.