Carson, D.A., and Douglas Moo An Introduction to the New Testament - Second Edition. Grand Rapids: Zondervan, 2005. "New Testament Letters" Carson & Moo pp. 331-353
p. 331 "The letter was not a typical method of religious instruction among Jews." So why do we find that no fewer than twent-one of the New Testament books are letters? p. 331 "The answer is probably twofold. First, the early Chrsitian movement, with its fast growth and peripatetic missionaries, demanded a means of communication at a distance . . . A second reason the letter may have been chosen by the apostles is its sense of personal immediacy."
NEW TESTAMENT LETTERS AGAINST THEIR GRECO-ROMAN BACKGROUND
The typical letter in the Greco-Roman world included an address/greeting portion, a body, and a closing, often sending greetings to others. Most of the New Testmaent letters follow this patten, generally with a more elaborate greeting in the form of a blessing or a doxology at the beginning. p. 333 "Classifications of ancient letters have their beginning in Adolf Deissmann's famous distinction between "epistles" (carefully composed, public pieces of literature) and "letters" (unstudied, private communications). Deissmann put all the letters of Paul into the latter category, arguing that they bore the same signs of hasty composition and lack of literary pretensions as are found in the Greek papyri letters. Deissmann's distinction was an artificial one, and it is now generally agreed that one cannot erect such rigid distinctions between a private letter and a public one."
THE USE OF AMANUENSES
An amanuensis is a scribe who would take dictation. It seems that these scribes were frequently used in the New Testament. For instance, in Romans 16:22 Tertius identifies himself as the one who wrote the letter. Typically the person who dictated the letter would add a final greeting after reading over the letter and approving it (cf. 2 Thess. 3:17 and Gal. 6:11). We do not know how much leeway authors gave to their amanuenses. In some cases it may have been considerable, in others we may have almost the exact words dictated.
THE COLLECTION OF PAUL'S LETTERS
THEORIES OF A SUDDEN COLLECTION
Many scholars consder that Paul's letters were collected into a group after a period of neglect, possibly fifty years or more after composition.
THEORIES OF A GRADUAL GROWTH
Other scholars consider that Paul's letters circulated and gradually were accumulated by different churches, being identified as a complete collection during the first century.
PSEUDONYMITY AND PSEUDEPIGRAPHY
Pseudonymity is the practice of identifying oneself as a different author, purporting that one's own work is that of another. Pseudepigraphy is similar, but involves simply placing a false title or superscription on a work. The terms are today used almost synonymously. Pseudonymous or pseudepigraphical works should be distinguished from apocryphal works, which would be viewed as works that contain error which would prevent their admission into a canon.
Pseudonymity is a widespread practice in antiquity. It is not always shunned and does not indicate a forgery. p. 338 "a literary forgery is a work written or modified with the intent to deceive. All literary forgeries are pseudepigraphical, but not all psuedepigrapha are literary forgeries: there is a substantial class of writings which, in the course of their transmission, became associated with some figure or other - judgments made with the best will in the world, however fallacious." Carson and Moo discuss multiple reasons why pseudepigraphers would ascribe their works to others.
Jewish literature is full of examples of texts which would be ascribed to someone else. It was not an uncommon practice. However, it was very uncommon in the writing of letters. A false claim to having written a letter would be considered fairly easy to detect.
EXTRABIBLICAL CHRISTIAN EXAMPLES
There are a good number of pseudonymous Christian works starting about the middle of the 2nd century. This includes some letters, including a collection of alleged correspondence between Paul and Seneca.
THE STANCE OF THE CHURCH FATHERS
Early Christian leaders were adamant in their opposition to works which they could find were forged or otherwise not authentic. This characteristic caution and desire to be public and honest in assessment of teaching which would be accepted as authoritative led to a higher standard of evidence among Christians than in the society at large.
EVIDENCE INTERNAL TO NEW TESTAMENT DOCUMENTS
p. 344 "Despite the consistent evidence from the early church outside the New Testament, many scholars assert, in the most confident terms, that writing letters in the name of another was common practice. Nowhere is evidence cited that any member of the New Testament church accepted the idea that a pious believer could write something in the name of an apostle and expect the writing to be welcomed."
SOME CONTEMPORARY THEORIES
p. 346 "Some are convinced that the New Testament contains many examples of literary forgeries and are unembarrassed by this conclusion."
p. 347 "On the other side are those who similarly point out how often deception plays a role in pseudepigraphy, but recall how the church universally rejected any hint of such deception."
p. 347 "In recent years several mediating positions have been advanced." These suggest that there may have been some instances of pseudonymity but that it was not widespread.