I thought this over, and came to the conclusion that the letter had once been meant for the post, but had been sent by hand.
Perhaps the writers may have occupied rooms in the same house.

The woman, in the anguish of her soul, may have sent the letter by a servant to her husband, and he, transported by rage, may have hurriedly scrawled this word across it, and returned it again: 'Take this to your mistress.' Having settled this point, I attacked the cipher, and, after fourteen hours' hard work, hit upon its meaning.
"Accidentally I held the piece of paper between myself and the light, with the side on which the writing was turned from me, and read it at once.

It was a cryptogram of the simplest kind, as the letters forming the words were simply reversed.

I divided the letters into words, and made out this sentence: '_Grace, je suis innocente.

Ayez pitie; rendez-moi notre enfant_ (Mercy, I am innocent.