Mezuzah check

The source for the mitzvah of mezuzah is from the torah in the book of Dvarim chapter 6 verse 19 "And you shall write them of the doorposts of your houses & gates".

The mitzvah is also brought in the Shlchan Aruch yorah Deah 285 The Halacha demands the checking of mezuzot every 3.5 years. Mezuzot even if kosher when purchased may be invalid with the passing of time. For this reason it is important to send them to be checked by qualified Magiah