James Joyce and his partner Nora Barnacle rather (in)famously exchanged very sexually-charged letters, many of which involved Joyce's feces fetish. If you'll forgive my linking to Buzzfeed, they have a quite entertaining sampling of some of the most salacious bits, such as

At every fuck I gave you your shameless tongue come bursting out through your lips and if I give you a bigger stronger fuck than usual fat dirty farts came spluttering out of your backside. You had an arse full of farts that night, darling, and I fucked them out of you, big fat fellows, long windy ones, quick little merry cracks and a lot of tiny little naughty farties ending in a long gush from your hole.

and

You say you will shit your drawers, dear, and let me fuck you then. I would like to hear you shit them, dear, first and then fuck you.

I'm sure others can post countless other examples of old-fashioned "sexting," and perhaps someone can provide a detailed analysis of the practice, but in terms of your "documented instances" question, that's the most famous example that comes to mind off the top of my head.

You may be interested in the 12th century letters of Abelard and Heloise.

Peter Abelard was a medieval French scholar of Logic and Theology at Notre Dame, said to have been preeminent in his day in these fields. At age 37, having thus far dedicated his life to the intellect and the Church, Abelard meets Heloise, a beautiful 19 year-old and herself a brilliant scholar of Latin, Greek and Hebrew, said to have been renowned throughout Europe for her scholasticism. Her uncle had arranged for her to take lessons under the famous Abelard. A romance soon develops. Heloise becomes pregnant. Her uncle, furious, breaks into Abelard's chambers with some hired assistants and castrates Abelard. In his shame and disgrace, Abelard decides to become a monk; however, he insists that Heloise do likewise and enter a nunnery, not wanting anyone else to know her as he has. The two now cloistered and separated from one another, this is where their famous correspondence begins. The two exchange a series of letters, expressing forcefully their passion to one another, and simultaneously their frustration at being kept apart and made to direct their love towards God as a substitute. Heloise, in her first letters, confides her continuing sexual memories of their relationship, and her writing betrays much pent-up sexual longing from her years of restraint. The letters continue, each less passionate than the previous, and then finally there is only silence between the two.

Less explicit, and therefore less directly relevant to your analogy of "sexting", were the poems Petrarch wrote for his unrequited love, Laura -- over 300 lyrical sonnets dedicated to the mysterious woman who was the object of his passion. Like Abelard, Petrarch had spent much of his early life in the pursuit of knowledge and in the service of the Church. But on leaving his vocation within the Church, he fell in love with the married woman, and throughout the rest of his life wrote very public love sonnets in her honor. So, perhaps his story might better be compared to a "Twitter" scandal, rather than a "sexting" chain.

Here is a link to the Project Gutenberg archive for the Letters of Abelard and Heloise

The Oxford Encyclopedia of Women in World History for the section on Heloise

This is a pretty great, if indulgent, lengthy analysis of Petrarch and Laura, from The Literature Network

One form of this worth considering was that apparently it was a thing for men to give women erotica in order to seduce them. In some cases, erotica was supposedly written for this purpose. The prime example of this is L'escholle des filles, or the School of Girls, which I write about in several blog posts (one, two, three). Samuel Pepys describes reading this book:

“I saw the French book which I did think to have had for my wife to translate, called L’escholle des Filles, but when I came to look into it, it is the most bawdy, lewd book that ever I saw, rather worse than Puttana Errante--so that I was ashamed of reading in it.”

(And then he bought it)

Regardless, this particular piece begins with this introduction:

None Madam can be a candidate with you for this dedication, ’tis your Lordship alone ha’s passed all the forms, and classes in this School, what delights you give, and with what eagerness you perform your Fucking exercises is sufficiently known to the many [who] have enjoyed you, for you Madam like the Supreame Powers have such a communicative goodness, as you scorn monopolizing your Cunt to a single keeper, but have generously refused no Man a kindness who desired it, having often been heard to say ’twas not in your nature to deny satisfaction to a standing Prick, and that ’twas not barely thrusting a Prick into a Cunt, but the well managing of a Fuck makes the Summum Bonum. [etc]

Following the instructions of the text to "give it into the hands of all girls" so that they may "become women," it appears that a man gave a copy of this legitimately raunchy text to a serving girl of Louie XIV. When it was discovered in her possession, the king had not only the "the particular maid of honour who was discovered to have the book expelled from her position, but also all the others (although the book was apparently given to her by a man rather than acquired by her own choosing)"

The book crops up elsewhere in the legal record--in particular, Massachusetts Bay Colony, where apparently a certain John Lancton spoke obscenely to a pair of women in the woods about the book:

"there he spoke of the book... and when I checked him he Laughed and he talked exceedingly uncleanly and Lasciviously so that I never heard any fellow Go so far after he was Gone we the the young women that were there agreed that we never heard any such talk come out of any mans mouth whatsoever it seem'd to me to be almost as bad as tongue could express"

The Library of Congress released [Warren G. Harding's naughty letters to his mistress] (http://www.huffingtonpost.com/2014/07/07/harding-love-letters_n_5564889.html) last year. He named his penis Jerry, suggesting his naming skills were on par with his ability to prevent corruption.