Mezuzah Found to be Not Kosher

Question:

I dutifully checked my mezuzot right before three and a half years had passed, as it says in the Kitzur. I was horrified to find that it had been damaged by humidity and was not kosher. I had no reason to suspect that the humidity would enter the mezuzah as it is in a properly sealed case in an inner room. Does this mean that from the time the damage occurred, I was not fulfilling my mitzvah? Should I have checked them more often? Every year? Every day? Also, do I need to fast or do some sort of teshuvah?

Answer:

It sounds like you did everything right:

You checked before the “three and a half years” point.^[1]
The mezuzah was not in a vulnerable place, such as in direct sunlight or exposed to the rain. In such cases, you would have had to check it at least once a year^[2] or more, depending on the circumstances.^[3]

As such, not only can you not be faulted for this unfortunate discovery, but you will receive reward as well. Our Sages assure:

If a person plans to do a mitzvah but is prevented from completing due to circumstances beyond his control, Scripture credits him as if he had performed it.^[4]

Although there is no halachic requirement to check indoor mezuzahs yearly, some authorities record the meritorious custom of checking mezuzot every Elul.^[5]

[1] Kuntres HaMezuzah, citing Yosef Ometz 479 and Parashah Sedurah 70.

[2] Aruch HaShulchan Y.D. 291:1.

[3] Mishnah Berurah 39:26, in reference to tefillin.

[4] Berachot 6a, cited in Agur B’ohalecha 39:6.

[5] Kitzur Shulchan Aruch 128:3; See Mateh Efraim 581:10.