Paul's letters were the first preserved documents of the emerging Jesus movement, predating the gospels by a generation or two. When he wrote his letters, he had no expectation that they would one day be accorded the status of Holy Scripture. With the exception of Romans, which was written to a community he had not yet visited, all the letters were written to churches he had recently founded as a response to specific issues that had cropped up in his absence.
His letters continued to be circulated in his communities after his death. By the second century, a group of ten were widely considered authoritative by most Christian churches scattered around the Mediterranean, but definitely not all. Remnants of Jewish Christianity considered his letters to be scurrilous attacks on traditional Israelite religion, but their views were in turn deemed heretical by the emerging orthodox consensus.
In the second century, three more letters called the Pastorals (1st and 2nd Timothy and Titus) were added to the Pauline collection, and many scholars of the day considered the anonymous letter to the Hebrews to be from Paul's hand, which helped Hebrews make the approved list of Christian books known as the canon. Current scholarship unanimously rejects Pauline authorship for Hebrews and a clear consensus rejects the pastorals as well, even though the author claims to be Paul. Scholars refer to the ancient practice of writing in the name of another as pseudopigrapha or even "pious fraud". Some current scholars suggest the Pastorals were written and falsely attributed to Paul in order to soften or correct his radical views, especially pertaining to women and slaves. Paul's bad rap as a misogynist is mostly unwarranted for this reason. Many current scholars would add Ephesians, Colossians, and 2nd Thessalonians to the list of letters falsely claiming Pauline authorship.
That leaves seven of the thirteen New Testament letters attributed to Paul as clearly authentic—albeit some are chopped up and pieced back together and there may be small accretions added here and there. The novel referenced four letters to the Corinthians, not two as indicated by the canon of the New Testament. Scholars agree that the Corinthian correspondence within the canon is the product of ancient cutting and pasting of multiple documents. Indeed, there may have been more than four letters from Paul to the Corinthians.
The remaining seven that are considered authentic are (listed in the possible order written) 1st Thessalonians, 1st Corinthians, Philemon, Philippians, 2nd Corinthians, Galatians, and Romans. All were penned in the years 50-56 CE (give or take a year or two), which in turn was a generation after the death of Jesus, which is commonly dated to 30-33 CE. For reference, the gospel of Mark is dated to 70 CE, Matthew to 85 CE, Luke (and Acts, its companion piece) to 85 CE or later, and John to 95 CE.
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