For Mary one of the cruellest aspects of her confinement was her isolation from the outside world caused by the increasing restrictions placed on her correspondence.
When Amyas Paulet took charge of her he was instructed to prevent all her correspondence save for that with the French ambassador in London, and even this he intercepted, read and either passed on or withheld as he saw fit. All Mary’s private mail, which had been reaching her thanks to the extracurricular activities of her laundresses was stopped. In the movie we see the subtle passing of messages, collected from the laundress by Mary’s maid Annette and slipped to the Queen, and the moment where Paulet confronts her with the news that this source of information has been discovered and cut.
Without news from the outside world Mary was of course delighted when a new source of secret mail began to arrive in the form of coded notes which were hidden in the bungs of the beer barrels that were sent to Chartley for her household. Indeed, in her craving for news, she was too hasty and failed to respond as cautiously as she had previously done to secret offers of help. In fact this route of communication was entirely compromised by Walsingham’s spy network and the messages were being passed to Thomas Phelippes, an expert cryptographer, who decoded them and passed the decrypts back to Walsingham. When Phelippes finished decoding Mary’s last, incriminating letter, he drew a gallows in the margin of the page. Mary’s fate was sealed.