Kiddushin, Chapter 4, Mishnah 3. In the first mishnah of this chapter we learned that a convert, a freed slave (a slave who was set free), a mamzer, a Nasin and one whose mamzer status is in doubt are all permitted to marry one another. Our mishnah broadens the ruling: even those who are forbidden to enter the congregation of Israel, such as an Ammonite convert or a Moabite convert, are permitted to marry mamzerim, a shetuki and an asufi. This is a novel point, since one might have thought that because they may not enter the congregation, they would likewise not be permitted to marry mamzerim, who are Jews.
The words of the Mishnah:
"Kol ha'asurin lavo bakahal mutarin lavo zeh bazeh" - anyone who is forbidden to enter the general congregation of Israel is permitted to marry another like himself. "Rabbi Yehudah oser" - Rabbi Yehudah forbids it.
The view of Rabbi Yehudah:
As the Gemara explains, Rabbi Yehudah forbids a convert to marry a mamzeres across the board. The root of the matter lies in the question of the "congregation of converts": the verse says, "A mamzer shall not enter the congregation of Hashem," and the question is whether the community of converts is also considered the congregation of Hashem like the rest of Israel, or whether it is a separate unit. Rabbi Yehudah holds that converts are considered the congregation of Hashem, and therefore a mamzer cannot marry into them.
Nevertheless, Rabbi Yehudah concedes in the case of an Ammonite or Moabite convert: such a convert himself may not marry into the rest of the Jewish people, and therefore he is not considered part of the congregation of Hashem, so a mamzer is permitted to marry him.
The view of Rabbi Eliezer:
Rabbi Eliezer addresses the case of doubtful mamzer status, and he distinguishes between certain and doubtful:
"Vada'an bevada'an - mutar" - a certain mamzer may marry a certain mamzeres, and the same applies to a Nasin.
"Vada'an besfekan, sfekan bevada'an, usfekan besfekan - asur" - a certain mamzer with one of doubtful status, one of doubtful status with a certain mamzer, and likewise one of doubtful status with another of doubtful status, are all forbidden.
It emerges that Rabbi Eliezer disagrees with what was stated in the first mishnah of the chapter, and holds that only a certain mamzer may marry a certain mamzeres.
And these are the doubtful cases:
Shetuki - one whose father is unknown.
Asufi - one who was gathered up from the street, and neither his father nor his mother is known.
Kuti - the Kutim are those people who converted at the end of the era of the first Beis HaMikdash. Even if we assume that they were full converts, in any event they were not careful in the laws of divorce and marriage, and therefore there was a concern that mamzerim were mixed among them, so they have the status of doubtful mamzerim.
In summary: the Mishnah teaches that all those who are forbidden to enter the congregation are permitted to marry one another, while Rabbi Yehudah forbids it, since in his view the congregation of converts is considered the congregation of Hashem, and he concedes in the case of an Ammonite or Moabite convert, who is not part of the congregation of Hashem. Rabbi Eliezer is stricter and permits only a certain mamzer with a certain mamzeres, forbidding any combination that involves doubt, and we detailed who the doubtful cases are: a shetuki, an asufi and a Kuti.