Packages from Overseas??

So my coworker tried to send post cards to her family in Poland and in the US, but they wouldn't send them out. Is the US just not accepting mail from South Korea? Or is South Korea the one refusing to send it? Does it go both ways? Because my family back in the states sent me a package over 2 weeks ago using express shipping and it still says it's in transit. Is South Korea not accepting packages from the US either??