From: Ya’akov, a slave of God and of the Lord Yeshua the Messiah.
Ya’akov was not only the half-brother of Yeshua and by now a slave of God and of the Lord Yeshua the Messiah and the leader of the Kehillah in Yerushalayim. We learn in Acts 15:13 ff. that he was also instrumental in sending a letter to the Goyim throughout Antioch, Syria, and Cilicia, instructing them to abstain from what has been sacrificed to idols, from blood, from things strangled, and from fornication. If you keep yourselves from these, you will be doing the right thing.
To: The Twelve Tribes in the Diaspora:
Now in his epistle, he turns his attention to Messianic Jewish Believers. The Twelve Tribes refers to Jews and is not merely a metaphor for Christians, as some Christian commentators maintain. This is clear from the style of the letter generally, particularly from the fact that they had synagogues, as we will see in chapter 2. Not that Gentile Believers were excluded from reading it, but that the leader of the Messianic Jewish community in Yerushalayim is addressing fellow Jewish Believers in the Diaspora, outside Israel.
The Diaspora became a technical term referring to Jews living outside the land of Israel. Besides the expulsions from the land by the Assyrians (2Ki 17; 1Ch 5) and Babylonians (2Ki 24, 25; 2Ch 36), many Jews were taken to Rome as slaves when the Romans conquered them around 63 BCE.
Yeshua instructs us: 12 When you enter someone’s household, say, ‘Shalom aleikhem!’ 13 If the home deserves it, let your shalom rest on it; if not, let your shalom return to you ~ Matthew 10:12-13 (CJB). The word shalom means not only peace but also tranquility, safety, well-being, welfare, health, contentment, success, comfort, wholeness, and integrity. Shalom aleikhem means “Peace be upon you” and is a standard greeting, as is “Shalom!” Therefore, there is a deeper meaning to Yeshua’s instruction in v. 13 on when to give or withhold shalom, for He refers not only to the greeting but to the whole complex of peace/wholeness/well-being that the Messiah offers through His talmidim and similarly at many places in the Brit Hadashah. 
We will learn a little more about Ya’akov as we dig into what he says about the Testing of Our Faith.
 Stern, D. H. (1996). Jewish New Testament Commentary.