Mezuzah Amulet for Protection
The Mitzvah of Mezuzah has a special property in that in its merit, Hashem protects the doorways of the houses of the Jewish nation from all harm. It is for this reason the name “Shaddai” is written on the outside of the Mezuzah, for this word is an acronym for “Shomer Daltot Yisrael,” meaning “Guardian of the doors of Israel.”
Hagaon Harav Moshe Feinstein zt”l in his Responsa Igrot Moshe (Yoreh De’ah, Volume 2, page 239) proves from the words of the Poskim that there is a certain “Segula” for protection by using a Mezuzah as an amulet.
(The word “Segula” appears in the Hebrew Bible in Exodus 19:5 and Deuteronomy 7:6, where God refers to the Jewish nation as His segula (treasure). The root of this word, segol, is the name of a Hebrew vowel-point represented by three dots. According to the Ohr Hachaim, a segula is "a charm that supersedes logic").