The Name שד"י Is an Acronym for ש'ומר ד'לתות י'שראל This Name on the Mezuzah Portends Protection of a Jew’s Home
I would like to propose a fascinating explanation as to why Yaakov Avinu chose to specifically employ the name Shakkai that is written on the mezuzah when praying on behalf of the shevatim. It is well-known that the mitzvah of mezuzah is a segulah to safeguard all the members of a household from all forms of evil and misfortune. The source for this is found in the Gemara (Menachos 43b):
"אמר רבי חנינא, בוא וראה שלא כמדת הקב"ה מדת בשר ודם. מדת בשר ודם מלך יושב מבפנים ועם משמרין אותו מבחוץ, מדת הקב"ה אינו כן, עבדיו יושבין מבפנים, והוא משמרן מבחוץ, שנאמר ה' שומרך ה' צלך על יד ימינך". Rabbi Chanina said: Come and see how unlike the characteristic of HKB”H is the characteristic of those made of flesh and blood. The characteristic of those of flesh and blood is that the king dwells inside and the people guard him from the outside. The characteristic of HKB”H is not so. Rather, his servants dwell inside, and He guards them from the outside, as it is stated (Tehillim 121, 5): “Hashem is your guardian; Hashem is your shade at your right hand.”
The Darkei Moshe (Y.D. 286, 4) writes in the name of the Shu”t Maharam of Rothenburg (108): "מובטח אני שכל בית שמתוקן במזוזה כהלכתה, אין שום מזיק יכול לשלוט בו"—I am certain that any house that is equipped with a proper mezuzah, no harmful force can prevail in it. This is also the message taught in the Gemara (ibid. 32b): "תלאה במקל או שהניחה אחר הדלת, סכנה ואין בה מצוה"—if he hung it in a rod or placed it behind the door, it poses a danger and there is no mitzvah in it. Rashi explains that there is danger from harmful forces, since the house is not afforded protection when a mezuzah is affixed improperly; protection is only conferred by a mezuzah affixed to the doorpost as required by halachah.
This is the rationale for the accepted Jewish minhag to write on the outside of the parchment the name שד"י; even though this minhag is not mentioned neither in the Talmud Bavli nor in the Talmud Yerushalmi. It is, however, mentioned in the Rambam (Hilchos Mezuzah 5, 4): It is a common custom to write שד"י on the outside of a mezuzah opposite the empty space left between the two passages. The Tur (Y.D. 288) also mentions this accepted practice citing his father, the Rosh. The Beis Yosef writes that the source for this minhag is found in the Zohar hakadosh (Vaeschanan 266a). And the Rama writes in Darkei Moshe (ibid. 3) in the name of the Kol Bo (90): "מה שנהגו לכתוב זה השם יותר משאר שמות, מפני ששם זה נוטריקון ש'ומר ד'ירת י'שראל"—the reason it is customary to write this name in preference to other names is because this name is an acronym for ש'ומר ד'ירת י'שראל—guardian of the Jewish home. In a similar vein, the Arizal explains that this name is an acronym for ש'ומר ד'לתות י'שראל—guardian of the doorways of Yisrael.
We find the following somewhat terrifying scenario portrayed in the Zohar hakadosh in the Raiyah Mehemna (Vaeschanan 263b): Next to the door stands a demon; it has permission to cause harm (to the people inside the house); it stands to the left of the doorway. When a person casts his eyes and sees the secret of the name of his Master on the mezuzah, which is on the right side of the doorway, and utters it, the demon cannot harm him.
The Chida writes in the marginal notes Nitzotzei Oros on the gloss of the Zohar (ibid. 4) in the name of the Arizal in Zohar HaRakia that the Zohar is referring to the divine name שד"י inscribed on the outside of the mezuzah. He concludes his comment as follows: "וצריך לכוון כי שם שד"י הוא שומר ישראל"—one should have in mind that the name Shakkai is the protector of Yisrael. Here are the words of the Arizal as brought down in the sefer Matzat Shimurim (Sha’ar Mezuzah), authored by the divine kabbalist Rabbi Nasan Shapira: And one must have in mind that the name שד"י stands for ש'ומר ד'לתות י'שראל—guardian of the doorways of Yisrael.
Now, the mitzvah of mezuzah is also a fantastic segulah for longevity. As the Tur writes (ibid. 285): "וכל הזהיר בה יאריכו ימיו וימי בניו, דכתיב למען ירבו ימיכם וימי בניכם, ואם אינו זהיר בה יתקצרו, דמכלל הן אתה שומע לאו, וכן דרשו חכמים בעון מזוזה בניו ובנותיו מתים קטנים". All that are diligent in the performance of this mitzvah will be rewarded with the prolongation of his life and the lives of his children, as it is written (Devarim 11, 21): “In order to prolong your days and the days of your children.” If one is not diligent in it, they will be shortened; from the positive ruling, you can infer the negative. Similarly, the sages taught (Shabbas 32b) that as a consequence of violating the mitzvah of mezuzah, one’s sons and daughters die young.
The source for this assertion is an elucidation in the Gemara (ibid.) regarding the juxtaposition of two pesukim in Krias Shema. First it states (ibid. 20): "וכתבתם על מזוזות ביתך ובשעריך"—the mitzvah of mezuzah—and immediately afterwards, it states (ibid. 21): "למען ירבו ימיכם וימי בניכם"—in order to prolong your days and the days of your children. This juxtaposition teaches us that the mezuzah is a segulah for long-life. Similarly, the michaber writes in the Shulchan Aruch (Y.D. 285, 1): One should pay careful attention to this mitzvah; all that are scrupulous in its performance, their lives and the lives of their children will be prolonged; if one is negligent in the performance of this mitzvah, their lives will be shortened.
Concerning this matter, we find a wonderful “remez” in the Shela hakadosh (Maseches Chullin) in the name of the Recanati (Mikeitz). He writes that for this reason, the word "מזזות" in the first passage of Krias Shema, in the passuk (ibid. 6, 9): "וכתבתם על מזזות ביתך ובשעריך"—is spelled deficiently, without its first “vav.” Spelled this way, it is an anagram for "זז מות"; this comes to teach us that in the merit of the mitzvah of mezuzah, the Shechinah dwells at the doorway entering one’s house; this saves us from the Malach Hamaves.
