Her love of scandal must have gone far to make her unpopular, and if her letters to her sister at Paris had been published she would have found herself with scarcely a friend in the world.
Correspondence between Lady Mary, from London or Twickenham, to her sister, the Countess of Mar, at Paris, was a very one-sided affair.

This was, in part, owing to the fact that Lord Mar was, of course, suspect, and that letters to him or to members of his family and household were (in all probability) intercepted in this country.

Lady Mary, who had suspected this more than once, became more and more convinced that her suspicions were justified.

"I have writ to you at least five-and-forty letters, dear sister, without receiving any answer, and resolved not to confide in post-house fidelity any more, being firmly persuaded that they never came to your hands, or you would not refuse one line to let me know how you do, which is and ever will be of great importance to me." That was written at Christmas, 1722, and though in the meantime Lady Mary heard from her sister, she realised that if she wanted her letters to arrive she must be careful as to the topics upon which she discoursed.

"Letters are so surely opened, I dare say nothing to you either of our intrigues or duels, both of which would afford great matter of mirth and speculation." The difficulties of communication did not decrease.