|It was common in the times of the Talmud for people to eat food with Sodomite salt (“Melah Sedomit”), which was known to be very caustic to the eyes. As such our Sages instituted the practice of washing one’s hands before reciting Birkat Hamazon so as to prevent salt from entering one’s eyes if one inadvertently touched them. This practice is known as Mayim Aharonim, or “last waters”.
The Tosafot (Hulin 105a) discuss the fact that Sodomite salt no longer exists and that perhaps Mayim Aharonim is therefore not necessary. The Rambam, however, says that even though the cause of the initial Rabbinic decree, that is the existence of Sodomite salt, is no longer present, the institution of Mayim Aharonim should remain intact. Additionally, the Meshech Hochma explains that each Rabbinic enactment is based on multiple reasons and that only some of them were revealed by our Sages. Furthermore, the Gemara teaches that there is a textual hint for Mayim Aharonim in the Torah. The Torah (Vayikra 11:44) says “And you shall sanctify yourselves, and you shall be holy”. “And you shall sanctify yourselves” refers to Mayim Rishonim, which is washing one’s hands before eating bread, and “and you shall be holy” refers to Mayim Aharonim. Based on this verse, the Vilna Gaon says that both washings are part of the same enactment, so just as Mayim Rishonim is still obligatory, so is Mayim Aharonim. As such, the Shulhan Aruch (Orah Haim 181:1) rules that Mayim Aharonim is obligatory, and uses the same expression as is commonly said “Mayim Aharonim Hova”. Although some communities are lenient in this regard, most rule like the Shulhan Aruch.
Another reason for Mayim Aharonim is to wash away any negative spiritual barriers (“Klipot“) from one’s hands. Consequently, the HIDA says that one should not use overly-copious amounts of waters such that the Klipot attach themselves to the water and drain back on one’s hands, but rather enough to wash from the knuckles down.
Regarding women and Mayim Aharonim, Rabbi Shalom Messas (Shemesh Umagen) writes that the custom in Morocco was to be lenient. This opinion is echoed by Rabbi Shmuel Wosner.
Finally it should be noted that if one is handling salt outside the context of a meal, such as measuring salt and the like, one is not obligated to wash one’s hands.
Summary: Mayim Aharonim is mandatory before Birkat Hamazon. Women do not have to wash Mayim Aharonim.