Misconception: There is a Talmudic source for the common practice of kissing the mezuzah upon entering and exiting a room.
FACT: There is no Talmudic source obligating one to kiss the mezuzah, although there may be a source for touching the mezuzah. Kissing the mezuzah seems to have been introduced by the Arizal (sixteenth century), and is thus a relatively recent custom.
BACKGROUND: Background: The mitzvah of mezuzah requires that a mezuzah be affixed to every doorway in one’s home. This is highlighted by the fact that one recites the berachah (“likbo’ah mezuzah”) at the time the mezuzah is affixed to the doorpost. From a halachic perspective, as long as the mezuzah is kosher and affixed to the doorpost, the mitzvah continues to be fulfilled without further active participation or acknowledgment on the part of the occupant. However, the Talmud alludes to the idea of having an ongoing “relationship” with the mezuzah (Avodah Zarah 11a):
Onkelos the son of Kalonymus became a proselyte. The emperor sent a contingent of Roman [soldiers] to pursue him, but he enticed them by [citing] Scriptural verses, and they converted to Judaism. The Emperor then sent another Roman cohort, instructing them not to say anything to him. As they were about to take him into custody, he said to them: “Let me tell you . . . ,” and they too converted. He [the emperor] dispatched another cohort and ordered them not to engage in any conversation with Onkelos. As they seized him and were walking, Onkelos saw the mezuzah affixed to the doorway. He placed his hand on it and asked them, “What is this?” They said, “You tell us.” Onkelos replied, “The universal custom is a mortal king dwells within and his servants keep guard over him from without; but with the Holy One, Blessed be He, His servants dwell within while He keeps guard over them from without, as it says, ‘Hashem yishmor tzetcha u’vo’echa me’atah v’ad olam, The Lord will guard your goings and your comings, from now and forever’” (Psalms 121:8). They too converted to Judaism. He [the emperor] sent for him no more.
This Talmudic story, citing Psalms 121:8 as support, links the mitzvah of mezuzah to the notion that God stands outside a Jewish home and protects those who dwell within. Elsewhere, the Gemara states that the mezuzah should be placed on the outermost tefach, handbreadth, of the entranceway (Menachot 33b; see Shulchan Aruch, YD 285:2). The rabbis offer various explanations for this rule, one of which is that a person should immediately encounter a mitzvah upon entering a house. Rabbi Chanina asserts that one should place the mezuzah on the outermost tefach so that the entire house will be protected. This is also how the Taz (YD 285:2) explains the halachah.
Rabbi Chanina adds that God’s protection is evident from a different verse in Psalms that alludes to the placement of the mezuzah (121:5): “Hashem shomrecha, Hashem tzilcha al yad yeminecha, The Lord is your guardian; the Lord is your shade at your right hand.” The mezuzah, which appears on the right side upon entering, protects the inhabitants.
The Rema in Darkei Moshe (YD 285), citing the Maharil, mentions the Onkelos story as the basis for the custom of touching the mezuzah. Note that while Onkelos touches the mezuzah, there is no mention of him kissing it.  In the Shulchan Aruch, the Rema also notes the custom of touching the mezuzah upon entering and leaving a house (YD 285:2). The Gra (YD 285:1) also assumes that touching the mezuzah is based on the Onkeles story, while the Aruch Hashulchan (YD 285:4) regards the Talmudic story as a weak source for the practice. Interestingly, Rav Yosef Hahn (d. 1637; Yosef Ometz 480) describes a copper mezuzah case with a copper cover over the name Shakai that could be rotated to allow a person to touch it with his finger.
Rabbi Akiva Eiger (Mahadura Kamma, Shu”t, no. 58) decries those who touch the mezuzah and do not differentiate between mezuzot with covers and those without, thereby sometimes inadvertently touching the parchment itself. Rabbi Eiger opines that touching the parchment is prohibited. He suggests that if a mezuzah lacks a cover, one should use his sleeve to touch it. While he was obviously familiar with the Talmudic tale involving Onkelos, he maintained that the custom of touching the mezuzah has no Talmudic basis.
The Onkelos story can be interpreted in one of two ways. It is possible that touching the mezuzah, as Onkelos did, was a standard practice at the time; thus, the Gemara provides evidence that almost 2,000 years ago it was customary to have an “ongoing relationship” with the mezuzah. Alternatively, Onkelos’ act can be viewed as an impromptu gesture to win over the imperial delegation (which was subsequently used as a basis for instituting the practice of touching the mezuzah).
Nowadays, kissing the mezuzah is a well-known practice as evidenced by its appearance in many twentieth-century seforim. For example, Chovat Hadar—citing the Chida who quotes the Arizal—states that one should kiss the mezuzah by placing one’s middle finger over the word Shakai, then kiss that finger and pray to God to be protected from the yetzer hara (Rabbi Yaakov Yeshaya Blau, 1976; p. 14). He also cites the Kuntres HaMezuzah which states that late, unnamed authorities also maintain that one should kiss the mezuzah. The Complete Mezuzah Guide states that “there is a dispute among the posekim whether one is required to kiss the mezuzah” (Rabbi Moshe Elefant and Rabbi Eliezer Weinbaum, ca. 1987; p. 19). In a footnote, it cites the Kitzur Shulchan Aruch (11:24) and the Misgeret Hashulchan which state that one is required to kiss the mezuzah when entering and exiting a house. Ohel Aryeh (Rabbi Label Katz, 1976; p. 121) cites the Onkelos story as well as Birkei Yosef which quotes the Arizal as stating that one should kiss the mezuzah with his middle finger (p. 123). The practice is so widespread that the late contemporary posek Rabbi Moshe Stern (the Debretziner Rav) deals with the question of which hand a “lefty” should use when kissing a mezuzah. He suggests that he use his left hand (Be’er Moshe 2:2:4). Indeed, the Steipler, who was a lefty, would kiss the mezuzah with his left hand. The Chazon Ish, however, would not touch the mezuzah, although he would look at it as he passed by (Orchot Rabbeinu, vol. 3, p. 164). Similarly, it is reported that Maharil Diskin did not touch the mezuzah but merely looked at it (Salmat Chaim 380). The leading Lithuanian halachic authority of the nineteenth century Rabbi Avraham Danzig says that when one leaves his house, he should kiss the mezuzah (Chayei Adam 15:1). The early twentieth-century Sephardic authority Rabbi Yaakov Chaim Sofer rules that when one leaves his house wrapped in his tallit on the way to shul, he should kiss the mezuzah (Kaf HaChaim, OC 25:22). The late Rabbi Dovid Lifschitz, a longtime rosh yeshivah at Rabbi Isaac Elchanan Theological Seminary, would touch the mezuzah as he passed it, according to one of his grandchildren.
Not all halachic authorities endorse the practice of kissing the mezuzah. Rabbi Yosef Eliyahu Henkin in Eidut Le’Yisrael (p. 159) objects to kissing the mezuzah (and sefer Torah) with one’s mouth or even with a cloth (and most likely with one’s hand as well). Instead he prefers the Sephardic, or more accurately, the Georgian (Soviet) custom of pointing and “blowing” a kiss. He offers two reasons for this. Firstly, he feels that kissing implies too much familiarity, a level of closeness that one cannot purport to have with a Torah or a mezuzah. Secondly, he opines that kissing a mezuzah even via one’s fingers or hand spreads germs, a hygienic-based halachic problem mentioned in Shulchan Aruch, OC 170:15. 
Kissing a loved or venerated object appears in other contexts in Jewish life. The Rema (OC 24:4) cites the custom of kissing tzitzit while reciting certain parts of Shema. The Kaf HaChaim (ibid. 19) notes that some people kiss the sukkah when they enter and exit and some kiss the arba minim; similarly others kiss the matzah and marror on Pesach. The Sha’arei Ephraim (Sha’ar 10, 4) says that when the Torah is taken out of the aron kodesh, those standing nearby kiss it and recite Shir Hashirim 1:2.
Possibly the practice of kissing the mezuzah symbolizes one’s desire for Divine protection as well as one’s love of God and His mitzvot. The Ktav V’Kabbalah (on Devarim 6:9) says that the mitzvah of mezuzah does not end with affixing the mezuzah to the doorpost. Rather, the mitzvah entails that one remain conscious of the mezuzah every time one enters and exits a room. To help attain this awareness, he says, the early authorities established the custom of touching the mezuzah whenever one passes it. So too, the Rambam (Hilchot Tefillin, Mezuzah 6:13) stresses the importance of being conscious of the mezuzah as one enters or exits a room.
Evidently, the custom of kissing the mezuzah is a recent one, and for most of Jewish history it was not a common practice. This awareness should in no way detract from the custom, but place it into perspective.
1. Excluding the bathroom and certain other rooms.
2. Thus the requirement to check one’s mezuzot twice every seven years (Yoma 11a; SA, YD 291:1) or even yearly (Kitzur Shulchan Aruch 128:3; Yechave Da’at 1:49).
3. Rashi says that this protection is from mazikin—“spirits.” Regarding mezuzah as a protector, see Yerushalmi Peah 1:1; Bereishit Rabbah 35:3; Yechezkel Lichtenstein, “Ha’Mezuzah K’segula L’shemirat Habayit,” Tchumin 10 (5749): 417-426. (I thank the late Moshe Tutnauer for showing this article to me.) See Daniel Sperber, Minhagei Yisrael, Vol. 2 (Jerusalem), 87. See Tosafot Yom Tov on Keilim 17:16 who speculates that in the Mishnaic period, travelers would take with them a mezuzah as protection and thought that it was a mitzvah. Mezuzah, more than most mitzvot, has “superstitious” elements associated with it as a segulah. The Tur (YD 285) says that the mezuzah guards the house, which the Beit Yosef explains is a clear miracle. See also the Rambam’s Hilchot Tefillin u’Mezuzah 5:4.
4. See Encyclopaedia Judaica 11:1474 for a fifteenth-century northern Italian illustration of a man touching the mezuzah as he leaves the house.
5. He also notes that one should say “Hashem yishmor tzeiti . . .” In Darkei Moshe, he only mentions the recitation of “Hashem shomri, Hashem tzali . . .” The Ben Ish Chai (Ki Tavo: 3) says that when leaving the house, one should put his hand on the mezuzah and say, “Hashem yishmor tzeiti u’vo’i l’chaim tovim u’lshalom, me’atah v’ad olam; Keil Shakai yevarech oti v’yitein li rachamim.” Despite the fact that he lived in contemporary times, he makes no mention of kissing the mezuzah.
6. Note that the Keset Hasofer (Rabbi Shlomo Ganzfried; Chakirah 19 [pp. 65b-66b in the 1980 ed.]) is lenient with regard to directly touching the mezuzah.
7. Birkei Yosef on SA,YD 285:2.
8. In a footnote, he quotes unnamed authorities who had the practice of kissing the mezuzah before going to sleep. The Kitzur Shulchan Aruch 71:4 also mentions this.
9. The Rema OC 149:1 mentions the custom of teaching children to kiss the Torah and the Kaf HaChaim 134:10; 149:10 notes the custom of adults kissing it. However, Rabbi Yosef Eliyahu Henkin is not alone in objecting to this custom. See also Pitchei She’arim 10:4 quoting the Kitzur Shelah and Siddur Tslusa d’Avraham, p. 375.
10. From 1903 to 1914, Rabbi Yosef Eliyahu Henkin was the rav of several different towns in Georgia. See Rabbi Yehuda Henkin, Equality Lost: Essays in Torah Commentary, Halacha, and Jewish Thought (Jerusalem, 1999), pp. 159-161.
11. On the dangers of kissing a mezuzah from a medical perspective, see Ilan Youngster et al, “Can religious icons be vectors of infectious diseases in hospital settings?” American Journal of Infection Control 37 (2009): 861-3.
12. If tefillin and tzitzit are looked at or touched/kissed while one is reciting the prayer of Shema, what about doing the same for mezuzah when it is mentioned in Shema? Seemingly, it is impractical to kiss the mezuzah when praying as it is not readily accessible (see Shu”t Rivash 486, cited in Beit Yosef, OC 24 as Rivash 2:126). See also the Kovetz Derushim (Warsaw, 1930) that contains Kuntres Toldot Ephraim by Rabbi Ephraim ben Rav Dov Ber from Kalish. In a sub-kuntres (Derech Efrat) he records notes about his father’s customs (p. 67). When his father davened at home and when he recited Shema before going to sleep, he kissed the mezuzah when it was mentioned in the prayer.
13. Rabbi Shlomo Zalman Auerbach (Halichot Shlomo, vol. 1, p. 89, n. 35) would use a retzuah (leather strap) to touch the tefillin and kiss it.
14. During a shiur he once gave in Beit Shemesh, Rabbi Dr. Abraham J. Twerski quipped, “I see people enter a room, kiss the mezuzah and then watch TV for a half hour. I would rather they kissed the TV and then watched the mezuzah for a half hour.”
Reprinted from JEWISH ACTION Magazine, Fall 2012 issue