Some scholars question whether 2 Thessalonians was written by Paul. Their arguments seem to be based on the belief that 2 Thessalonians is too similar to 1 Thessalonians while also suggesting that 2 Thessalonians is too different from 1 Thessalonians (and other Pauline epistles). Online, Dr. Daniel Wallace provides a perusasive argument for its authenticity. As his article makes clear, the external evidence for the autheniticity of 2 Thessalonians is very strong:
Not only is 2 Thessalonians found in Marcion’s canon and the Muratorian canon, but it is also quoted by name by Irenaeus, and was apparently known to Ignatius, Justin Martyr, and Polycarp. Further, it is found in the most ancient MSS (including the old Latin, old Syriac, and p46), suggesting its full acceptance from a very early period. Although not as strong as the evidence for the Hauptbriefe (in terms of frequency of citation), 2 Thessalonians has nevertheless enjoyed universal acceptance. In fact, the external testimony for 2 Thessalonians is equally as strong as, if not stronger than, that of 1 Thessalonians.
Dr. Wallace also provides persuasive points responding to the arguments from the internal evidence. However, I recently finished reading the introduction to I. Howard Marshall's 1 and 2 Thessalonians, A Commentary, and wanted to bring it to the attention of those interested in this issue. Prof. Marshal provides an able but quite readable response to the usual arguments against autheniticity. See Marshall, 1 and 2 Thessalonians, pages 28-45. It is relatively inexpensive as commentaries go, and has the added benefit of covering both books. Marshall brings welcome common sense to the issues of the literary structure, theoligical arguments, and the situation of the letter.