The letters of Paul

Undisputed letters

  • Romans
  • 1 Corinthians
  • 2 Corinthians
  • Galatians
  • Philippians
  • 1 Thessalonians
  • Philemon

These seven letters are quoted or mentioned by the earliest of sources, and are included in every ancient canon, including that of Marcion. There is no record of scholarly doubt concerning authorship until the 19th century. -- Wikipedia

Deutero-Pauline Epistles

The difference in writing style, use of language and introduction of new words causes many scholars to dispute whether Paul was the genuine author of these books.

However, some suggest these are weak arguments and suggest that these books might have been written on behalf of Paul by his disciples.

Pastoral Epistles

The following books are considered the pastoral epistles:

These books writing styles and use of language differs from the other epistles written by Paul, which means that Paul might not be the author although the books claim that he is.

More reasons why some dispute these books' authenticity:

Norman Perrin argued that Paul's travels to Crete (Titus 1:5-6), again to Ephesus (1 Timothy 1:3), Nicopolis (Titus 3:12), and Troas (2 Timothy 1:15, 4:13) cannot be fit into any reconstruction of Paul's life or works as determined from the other epistles or from Acts.

...

Other reasons for a 2nd-century date have been argued. The Pastoral Epistles lay out church organisation concerning the character and requirements for bishops, elders, deacons, and widows. Some scholars have claimed that these offices could not have appeared during Paul's lifetime. In terms of theology, some scholars claim that the Pastorals reflect more the characteristics of 2nd century (Proto-orthodox) church thought, than those of the 1st century.

...

Lastly, some have argued that the Pastorals condemn forms of Hellenic mysticism and gnosticism, which were seen as not significant in the 1st century.

-- Wikipedia

However, many scholars consider these arguments weak and that these arguments do not necessary proof anything.