There are six factors that prevent

There are six factors that prevent [a priest] from reciting the priestly blessings: [an inability] to pronounce [the blessings properly], physical deformities, transgressions, [lack of] maturity, intoxication, and the ritual impurity of [the priest's] hands.

[An inability] to pronounce [the blessings properly]: What is implied? Those who cannot articulate the letters properly - e.g., those who read an aleph as an ayin and an ayin as an aleph, or who pronounce shibbolet as sibbolet and the like - should not recite the priestly blessings.

Similarly, a stutterer or one who speaks unclearly, whose words cannot be understood by everyone, should not recite the priestly blessing.

Halacha 2
Physical deformities: What is implied? A priest should not recite the priestly blessings if he has blemishes on his face, hands, or feet - for example, his fingers are bent over, crooked, or covered with white spots - for they will attract the people's attention.

A person whose spittle always dribbles when he speaks, and also a person who is blind in one eye should not recite the priestly blessings. However, if such a person was well known in his city and everyone was familiar with the person who was blind in one eye or whose spittle dribbled, he may recite the priestly blessing, for he will not attract their attention.

Similarly, a person whose hands were colored purple or scarlet should not recite the priestly blessings. If the majority of the city's population is involved in such a profession, he is permitted, for this does not attract the people's attention.

Halacha 3
Transgressions: What is implied? A priest who killed someone should never recite the priestly blessings, even if he repents, as [implied by Isaiah 1:15 which] states: "Your hands are full of blood," and states: "When you spread out your hands, I will hide My eyes from you."

A priest who served false gods, even if he was compelled to do so or did so inadvertently - though he has repented - may never recite the priestly blessing, as [can be inferred from II Kings 23:9, which] states: "However, the priests of the high places shall not ascend [to God's altar in Jerusalem]." [The recitation of the priestly] blessings is equated to service [in the Temple], as [Deuteronomy 10:8] states: "to serve Him and to bless in His name."

Similarly, a priest who converted to the worship of false gods - even though he repents - may never recite the priestly blessing. Other transgressions do not prevent [a priest from blessing the people].