Did Paul write other letters that are not in the Bible? I've heard it said that Paul wrote other letters that didn't make it into the Bible, including at least one more letter to the church at Corinth, and a letter to the church at Laodicea. What is the source for these claims, and what might have happened to these letters?
Paul certainly wrote other letters, but they were either lost or were not theological. For example, 1 Corinthians 16:3:


  Then, when I arrive, I will give letters of introduction to the men you approve and send them with your gift to Jerusalem.


Regarding one more letter to Corinth, that is the implication in 1 Corinthians 5:9 when he refers to an earlier letter:


  I wrote to you in my letter not to associate with sexually immoral people.


Finally, it is worth noting that all genuine letters by Paul could be identified by people at the time as he ended them in his own writing (rather than dictating). 2 Thessalonians 3:17 says:


  I, Paul, write this greeting in my own hand, which is the distinguishing mark in all my letters. This is how I write.