We should be careful from having any kind of dirty things or impurities by the opening of our homes. Water that washed the dirty floors inside should not be pours outside the door for the reasons that it would desecrate the name of God on the Mezuzah. Impurities by the door leave it without protection and the demon is allowed to inflict harm. The opening to the house should always be clean to respect the name and have our protector present at all times.
When the Mezuzah is properly installed on the door post, the evil inclination and the demon at the door praising the people saying;
“זֶה הַשַּׁעַר לַיהוָה צַדִּיקִים יָבֹאוּ בוֹ”
“This is the gate of YHVH; the righteous shall enter through it.”
When there is no Mezuzah then the demon and the evil inclination get greater strength. When a person comes to enter the door they put their hands on his head and say “Wow to this person that left the protection of his master”. From that point that person is without protection and depend only to the mercy of God.
Daily Zohar # 2801 – Vaetchanan – The protection of the Mezuzah – 3