The confidence that some interpreters have about what Paul knew, or used, literarily (e.g., rhetorical handbooks and the like) has puzzled me. I am bit more comfortable with the following approach.
"It is difficult to envision Paul sitting down with a dog-eared copy of Pseudo-Demetrius or Pseudo-Libanius as he scolds the Galatians or answers questions posed by the Corinthians. Elements of most of the letter types listed in the handbooks are nevertheless present in his letters. These types are akin to the basic chords, rhythms, and harmonies that a skilled musician varies, builds on, amplifies, and inverts in improvisational jazz. However spontaneous it may sound, they do not create new music out of thin air or without mastery of the fundamentals. Paul is likewise fluent in the fundamentals of letter writing. The way he manipulates, combines, conforms to, or subverts the familiar genres and subgenres displays his literary virtuosity."
Patrick Gray, Opening Paul's Letters: A Reader's Guide to Genre and Interpretation (Grand Rapids: Baker, 2011), 52.