What is the significance that Hash-m guards His servants from the outside?

Iyun Yakov: This shows His humility. His servants sit, and He stands to guard them! So it says about Avraham "Hu Yoshev", and the Shechinah stood; "va'Yar Elav Hash-m" - "v'Anvascha Sarbeni" (Bereishis Rabah 48:1). It says about Yakov "va'Shem Nitzav Alav Leshamro." This is even if they do not do His will so much, for they are called Avadim; when they do His will, they are called Banim (Kidushin 36a). He can be so humble to pardon His honor, for the entire world is His. The Mizmor begins "Ezri me'Im Hash-m Ose Shamayim va'Aretz."

Daf Al ha'Daf citing Klei Golah: Chachamim do not need guarding (Bava Basra 8a). Torah shields and saves (Sotah 21a). This is why R. Chanina switched from 'Am' (regarding a mortal king) to 'Avadav', to show that Mezuzah is needed to guard only those who do not do His will so much. (NOTE: The text in Ein Yakov says 'Avadav' also regarding a mortal king. - PF)

What is the source that "Hash-m Shomerecha Hash-m Tzilecha Al Yad Yeminecha" refers to Mezuzah?

Rashi: Mezuzah is on the Yemin (right) when one enters.

Maharsha: The Tehilim continues "Yomam ha'Shemesh Lo Yakecha", for all evil occurrences come to the world from the Mazalos; the primary ones are the sun, moon and their stars. They oppose Yisrael - 'Ein Mazal l'Yisrael' (Shabbos 156a). (NOTE: Maharsha explains 'EIn Mazal l'Yisrael', i.e. Mazal opposes Yisrael. Rashi there explained that Yisrael are able to change their Mazal via Tefilah and merits. - PF) Due to Mitzvas Mezuzah, Hash-m will guard you, and the sun and moon will not strike you. Do not say that this is only when you enter your house. It is also when you leave - "Hash-m Yishmor Tzeisecha u'Vo'echa..."

Etz Yosef: Shakai written on the outside of the Mezuzah is an acronym for Shomer Dalsos Yisrael [not to let the destroyer wound in a Yisrael's house].