rationalskepticism.org

The name "undisputed" epistles represents the traditional scholarly consensus asserting that Paul authored each letter. However, even the least disputed of letters, such as Galatians, have found critics11; Moreover, the unity of the letters is questioned by some scholars. First and Second Corinthians have garnered particular suspicion, with some scholars, among them Edgar Goodspeed and Norman Perrin, supposing one or both texts as we have them today are actually amalgamations of multiple individual letters. There remains considerable discussion as to the presence of possible significant interpolations. However, such textual corruption is difficult to detect and even more so to verify, leaving little agreement as to the extent of the epistles' integrity. See also Radical Criticism, which maintains that the external evidence for attributing any of the letters to Paul is so weak, that it should be considered that all the letters appearing in the Marcion canon were written in Paul's name by members of the Marcionite Church and were afterwards edited and adopted by the Catholic Church.

http://en.wikipedia.org/wiki/Authorship_of_the_Pauline_epistles#The_.22undisputed.22_epistles

11 A. Q. Morton and J. McLeman, Paul, the Man and the Myth (1966).

Radical_Criticism is a movement around the late 19th century that, typically, denied authentic authorship of the Pauline epistles.

The Dutch school of radical criticism started in 1878 with a publication by Allard Pierson, who denied Pauline authorship of Galatians. He was fiercely attacked by his colleague A.D. Loman, but two years later Loman also abandoned the historicity of Paul. Similarly, W.C. van Manen, who had written a doctoral thesis defending the authenticity of 1 Thessalonians, wrote in 1889 that he had come to the same conclusions as Loman.
.