The Epistle to Seneca the Younger is a collection of correspondence claiming to be from Paul of Tarsus to Seneca the Younger. There are 8 epistles allegedly from Seneca, and 6 replies allegedly from Paul[1]. However, it is widely held to be forged, even in early times, in particular as the styles of writing match neither Paul's other epistles, nor Seneca's other works.
Seneca is said to have been concerned about the allegorical style of Paul's epistles and to have coached him on the appropriate manner of writing. Such an appeal to as great a writer as Seneca is clearly an attempt to lend the theological content of the epistles, which was that the Pauline Epistles are allegory encoding secret teachings, some weighting.
Jerome mentioned them in his De Viris Illustribus (chap. 12)[2].