Q:

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?

A:

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.

A:

According to John MacArthur in his sermon titled Saved or Self-Deceived (available on YouTube here), he says Paul wrote two more letters to the church of Corinth (about 19 minutes into the sermon). 
He explicitly says that they were not included in the Bible. "..a church to which by the time he writes 2nd Corinthians he's already written three other letters, 1st Corinthians and two other letters that aren't in Scripture." John Mac Arthur. As one of the most authentic bible scholars of our time, I would take his word for it.

A:

Jerome Murphy-Oconnor says:

The intensity of Paul's relationship with the Corinthians is illustrated by the fact that he wrote more letters to them than to any other church. The New Testament contains only two letters, but these mention two others, the Previous Letter (1 Cor. 5: 9) and the Painful Letter (2 Cor. 2: 4). Hence, four in all.
From the end of the eighteenth century, however, doubts have been raised regarding the integrity of both i and 2 Corinthians. 2 The division of 2 Corinthians into two originally independent letters was postulated in 1776. It took a hundred years for the integrity of 1 Corinthians to be called into question. From that moment hypotheses became ever more complex as fragments from one letter were associated with those from another. This trend in New Testament research reached its climax with the thesis that originally the Corinthian correspondence consisted of nine distinct letters.

Sources

Jerome Murphy-O’Connor, Paul: A Critical Life, 1st edn (New York: Oxford University Press, 1998).p.252

See also Schmithals (1973). For a survey of the various modern partition theories, see Sellin (1987), 2965-8.

