Epistolary structure of a biblical book

A biblical book written in epistolary genre has the following structure: 

• a brief introduction, 
• a personal conclusion, 

and in between -- two major segments:

• a doctrinal section, followed by 
• a practical section.

Examples: Ephesians, Colossians, other Pauline writings.

Noted from the Synthetic Bible Study series by Howard Hendricks (Prof) on www.discipleshiplibrary.com