An Edomite cannot marry a Jew for three generations.
Nowadays (since the time of Sancheiriv, who mixed all nations together), this law no longer applies as we no longer know who is an Edomite. However, there were people who had a tradition of their lineage. For example, Haman was a known Amaleiki,. If someone had a tradition that they were one of the "original Edomites" or "original Moabites", would he be prohibited to marry a Jewess?