Heidegger's Bible Handbook: Hebrews: Inscription
1. Why was the Epistle inscribed to the Hebrews? The Hebrews are understood as whichever ones indiscriminately? The Inscription is referred to the author himself, and received with good reason.
This Epistle is inscribed, ἡ πρὸς Ἑβραίους Ἐπιστολὴ, the Epistle to the Hebrews. Namely, an Epistle, because it was sent to those for whom it was principally written. But, to the Hebrews, because it was written to Hebrews, and intended for them; whether be understood those that were dwelling at Jerusalem, or at least in Palestine, called Hebrews after the ancient appellation of their race, Acts 6:1; or Hellenists, or Grecian Jews dwelling on the near side of Jerusalem, called Hellenists because they all or most were speaking Greek, and in their Synagogues were reading, not the Hebrew Bible, but the Greek; although Salmasius insists that those were called Hellenists, that were of a foreign nation, and so were converted to the Jewish religion as proselytes: or, finally, all the Churches gathered of the Jews, and so all the Jews indiscriminately, whether in Palestine, or in Syria, or in other lands, which is made all the more likely, by the notion of Hebrews, signifying all Israelite Jews; and by the argument of the Epistle, concerning all Jews indiscriminately. For, what is promised in Hebrews 13:23, that he is going to see the Hebrews, is to be understood, not definitely, but indefinitely, in this sense, that he, being freed from his bonds, was going to see some of them, and perhaps those specifically to whom the Epistle was first sent. Moreover, this inscription, since it is extant in all the most ancient Greek codices and versions, and is mentioned by Tertullian and the later Fathers in so many words, is referred to the very author of the Epistle, and is with good reason received.
Ἐπιστολὴ/epistle is related to the verb ἐπιστέλλω, to send to.
Dr. Dilday's Lecture: "A Pauline Miscellany, Part 2"