Upon finding out about Mary Alice having "stolen" Deirdre Taylor's baby boy, Martha Huber sent a blackmail note, bluntly saying "I KNOW WHAT YOU DID. IT MAKES ME SICK. I'M GOING TO TELL". Martha was hoping to use this note to blackmail money out of Mary Alice, but Mary Alice instead shot herself. The letter stayed hidden in her cupboard.
When Bree, Gabrielle, Susan and Lynette helped Paul clean out some of Mary Alice's belongings, they came across the letter. Gabrielle opened it, despite Bree telling her not too. They read it and then knew Mary Alice had a dark secret.
A while later, Martha confessed everything to Paul and said that she was the one who sent the letter because she needed money and it was better to take it from a bad person, which ultimately lead to Paul strangling her with Mary Alice's blender.
In the last season, Orson Hodge sent Bree that same note, with the intent of winning Bree back. This didn't work though, because, in the fifteenth episode, Bree learns of this after seeing a picture of them on that one horrible night they covered up the murder, and, after realizing what Orson is doing, everything refuses to travel to Maine with him. In retaliation, Orson reveals everything to the police, setting a chain of events in motion.