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It is proper during the month of elul to include in the heading of every personal letter a wish for a good year, such as "Le’shana Toba Tikateb" ("May you be inscribed for a good year"). Today, when people send dozens of emails every day, we might want to consider having such a greeting automatically appear at the top of our email correspondences during the month of elul, in accordance with this Halacha.
There is also a custom among especially righteous Jews to have their Tefillin and Mezuzot checked each year during elul. According to the strict Halacha, Mezuzot must be checked once every three-and-a-half years. Even though the Mezuzot were confirmed valid when they were purchased, and they remained untouched on the doorpost inside a case ever since, they still require checking once every three-and-a-half years. (An advisable way to remember to check the Mezuzot is to check them every leap year.) However, there is a laudable practice on the level of Midat Hasidut (an extra measure of piety) to have the Mezuzot checked each year during the month of elul.
As for Tefillin, one is not required, according to strict Halacha, to have his Tefillin checked at all. Tefillin have a presumed status ("Hazaka") of validity once they were confirmed valid at the time of purchase, and thus do not require checking. The Tefillin produced today are written on "Gasot," thick leather, which is far more durable than the thin "Dakot" parchment used long ago. We may therefore presume the validity of our Tefillin even without periodically checking them. The only time one must check his Tefillin according to Halacha is if he treated them negligently, such as if he wore them with wet hair, which could ruin the parchment, or if he left his Tefillin inside his car on a hot day. In such cases, one must have his Tefillin checked to ensure that they were not damaged. Otherwise, Tefillin do not require checking. Nevertheless, there is a custom on the level of Midat Hasidut to have one’s Tefillin checked each year during the month of elul as part of our spiritual preparations for Rosh Hashanah.
Summary: It is proper to include a wish for a good year in the heading of written correspondence (including email) during elul. Strictly speaking, Mezuzot must be checked only once every three-and-a-half years, and Tefillin do not require checking at all (unless they were treated negligently). Nevertheless, there is a laudable practice to have one’s Tefillin and Mezuzot checked each year during elul.