The Gemara in Megillah 18b records a difference of opinion whether a mezuzah requires sirtut – whether horizontal lines must be scratched into the parchment’s surface for the words to be written upon. There is no doubt that Torah scrolls, megillot, and even divorce documents are require sirtut, and that tefillin do not. What is the source of doubt with regard to mezuzot?
There are several opinions amongst the Rishonim (medieval commentators) as to the purpose of a mezuzah. According to Rambam and the Sefer Ha-Chinuch (and a Midrash in Parshat Korach), the purpose of the mezuzah is to remind us of God’s Presence as we enter our homes. On the other hand, the Zohar and the Talmud Yerushalmi indicate that the mezuzah serves as a type of protection for the home.
This dispute about the purpose of the mezuzah helps explain the difference of opinion regarding sirtut. Writing on lines, as we all know from grade school, makes the writing legible. Legible writing is desirable if it to be read. Tefillin, which are never read, need no sirtut. Torah scrolls, megillot, and gittin must be legible because they are read.
With regard to mezuzot, if they serve as protective amulets, then there is no need for them to be written legibly. If, however, they help define what makes this house Godly, what one can expect as they cross the threshold into a Jewish home, then ideally the text should be readable to all who pass.
One can tell a lot about a place by the messages one encounters upon entering and exiting. There are neighborhoods where, upon entering, one is instructed how to dress appropriately. There are cities which proclaim themselves the ‘City of the Future’.