It is important to know the epistolary genre of the Greco-Roman world when analyzing the structure of the letters of the New Testament. A letter written in the Greco-Roman world tended to consists of the following parts.
The author would usually introduce his letter.
1. Salutation (Sender; receiver; greeting)
An author would begin by identifying himself as the sender and then identify his intended reader(s). Following this, he would extend a greeting to his intended readers.
After the salutation, the author would offer a prayer or a thanksgiving on behalf of his intended readers. The gods/God to whom the author would pray or give thanks would depend upon the author's religious and cultural background.
B. Main body of the letter
Following the introduction, the author would deal with the matter for which he is writing. This could vary from the mundane to the sublime, depending on the author's purpose.
C. Conclusion (Greetings; benediction/farewell)
Typically, in order to close a letter, the author would send greetings
to and greetings from people known to both him and the reader(s). He would
also give a benediction and a farewell to his reader(s).