Ancient letter writers writing open letters?

I’ve been going through some old books from college and noticed that some of the ol’ Romans (Cicero is the only one whose name I can remember, but I think Seneca was in there was well) wrote some surprisingly long and pointed letters. These have, of course, been passed on through the millennia and published at various points.

When they were writing these letters (Cicero’s attacks on his senatorial opponenets for example), were they intended as “open letters” or were they simply something to pass the long hours?

I’m sure it depends on the individual, but there are enough of these things that I can’t help but think they were writing for an audience.

Generally they actually were letters written to another person; many of them include Latin salutations common at their time (for example S.V.B.E.E.V., si vales, bene est, ego valeo - if you’re healthy, that’s good, I’m healthy). How they ended up being recorded is another question - I guess the recipient archived letters he considered of literary value, maybe passed them on, and somehow found their way into collections that were copied manually again and again throughout the middle ages.

It should be added that antiquity did not have anything comparable to our notion of intellectual property; so a writer might have anticipated that the recipient was going to make the work public and therefore it was, in the end, indeed written for an audience although addressed to an individual. My guess.

Specifically, most of Cicero’s letters were addressed to one of three people; his brother Quintus, who served with Caesar in Gaul and was a partisan of his, his friend Atticus, who was a Roman nobleman living in Athens who stayed out of politics, and focused almost entirely on getting rich, and Tiro, his slave and later freedman.