One other thing to consider:
There may be some letters that, though written to a specific church, contain instructions and things of importance to all churches.
And there may be letters written to a specific church that were not written in that way.
We know there were specific instructions given for letters to be spread to other churches
Col 4:16 And when this letter has been read among you, have it also read in the church of the Laodiceans; and see that you also read the letter from Laodicea.