Is the book of Revelation a Letter or Apokalypse (ἀποκάλυψις)? Because the Book of Revelation has the characters of an Epistel eg. Prescript, Sender, Receiver, Thanking and even with Final greetings at the end.
The two are not mutually exclusive.
According to LSJ,1 an epistle/letter (ἐπιστολή) is “anything sent by a messenger, message, order, commission, whether verbal or in writing.”
In Revelation 1:1, it is written,
1 The Revelation2 of Jesus Christ, which God gave Him to show His servants—things which must shortly take place. And He sent and signified it by His angel to His servant John, NKJV, ©1982
Αʹ Ἀποκάλυψις Ἰησοῦ Χριστοῦ ἣν ἔδωκεν αὐτῷ ὁ θεὸς δεῖξαι τοῖς δούλοις αὐτοῦ ἃ δεῖ γενέσθαι ἐν τάχει, καὶ ἐσήμανεν ἀποστείλας διὰ τοῦ ἀγγέλου αὐτοῦ τῷ δούλῳ αὐτοῦ Ἰωάννῃ, NA28, ©2012
The apocalypse/revelation is the message, or thing sent by messenger. It originates with God the Father, who gives it to Jesus Christ, who sends and signifies it by his angel, who delivers it to John, all of which do not occur in writing.
Then, after John receives it from the angel (or messenger), he then writes it on parchment (in a “book” or roll)3 and sends it to the seven churches which are in Asia.4
1 LSJ, p. 660
2 or “apocalypse”
3 Rev. 21:5 cf. 22:18–19
4 Rev. 1:4
Liddell, Henry George; Scott, Robert; et al. A Greek-English Lexicon. 9th ed. Oxford: Clarendon, 1940.