(20) Behold, I send an angel before thee, to keep thee by the way, and to bring thee into the place which I have prepared. (21) Take heed of him, and hearken unto his voice; be not rebellious against him; for he will not pardon your transgression; for My name is in him. (22) But if thou shalt indeed hearken unto his voice, and do all that I speak; then I will be an enemy unto thine enemies, and an adversary unto thine adversaries.

(2) He is not accustomed to that [i.e., forgiving], for he is of the group that do not sin. And moreover, he is a messenger, and he can do only his mission. -[From Midrash Tanchuma 18]
(3) This is connected to the beginning of the statement that My Name is with him. And our sages said that this is the Angel Metatron, whose name is that of his Master, Metatron is the numerical value of Shadday.

Another explanation: Behold, I send an angel before you...It is written there (Jeremiah 3:19: But I said: 'How would I put you among the sons, and give you a pleasant land, the goodliest heritage of the nations!' What does it mean: 'put you'? 

The Holy Blessed One said: From the moment you stood at Sinai, and accepted the Torah--I wrote that I loved you, as it is written (Deuteronomy 7:8): But because Adonai loved you; once I have loved you, how can I hate you?...

Thus the Holy Blessed One said to Israel: Did I cause you to lack anything, that you rejected Me? Hence, (Jeremiah 3:20): Surely, as a wife treacherously departs from her friend, etc... Another explanation of Behold, I send an angel before you...Had you merited it, I myself would have become your messenger, just as I was in the wilderness, as it says (Ex. 13:21): And Adonai went before them by day...but now that you have not merited this, I am entrusting you into the hands of a messenger, hence: Behold, I send an angel before you...

Behold, I am sending an angel before you...Here is it foreshadowing that they are destined to sin, and the Divine Presence says that it will not go in their midst (as in Ex. 33:3)

For my name is in your midst: This is related to the first part of the verse: Take heed of him, for My name is shared within him. And our Sages teach that this is Metatron, who name is our Master's. Metatron in gematria is equal to Shadai--this is Rashi's language. We also see in Exodus Rabbah, that there were those who asked for this because of the Golden Calf incident. But there were also those who asked that this decree not be carried out.

It says (Ex. 33:2, 3): I will send an angel before you....I will not go in your midst. And Moses asks for mercy, and says (Ex. 33:15): Unless You go in the lead, do not make us leave this place...

And the solution is thus, according to this understanding--the decree was not carried out in Moses' lifetime...but after his death, God did send an angel. And this is the meaning of the verse (Joshua 5:13): Once, when Joshua was near Jericho, he looked up and saw a man standing before him, drawn sword in hand....The purpose of this vision was to show Joshua that the angel was sent to go into battle before them.

But, the true understanding is that this angel whom they are promised is the redeeming angel of Genesis 48:16, who has God's name in him--For, in Yah Adonai, you have an everlasting Rock (Isaiah 26:4)...

The text calls him an angel, because the whole world is managed under the divine aspect that he embodies. And our Sages say this is Metatron, and that this is the name of the guide.

Once, a heretic said to Rav Idith: It is written (Exodus 24:1): And to Moses, he said: Come up to Adonai. It should have said: Come up to Me! He said: It was Metatron, who name is similar to that of his Master, as it is written (Exodus 23:21): For My name is in him. But if so, we should worship him!! It is written (Exodus 23:21): Be not rebellious against him--i.e. exchange me for him. But if so, why is it stated (Exodus 23:21): He will not pardon your transgression? We hold the belief that, we would not accept him even as a messenger, for it is written (Exodus 33:15): He said to God: Unless You go in the lead, do not make us leave this place...