How does paul young from desperate housewives know Martha was blackmailing Mary elice?

Answer 1

After Mary Alice's suicide, Paul held a yard sale to get rid of his wife's clothes and some of her personal effects. The other housewives helped him get ready and one found an anonymous blackmail note in Mary Alice's things. The envelope was postmarked with the date of MA's death.

Several epidsodes later, the housewives came clean and turned the note over to Paul Young; he hired a seedy P.I. to find the author of the note. Turns out that Edie Britt had also given a handwritten note to Paul; both Edie's note and the anonymous blackmail letter were on written on the same type of stationary.

Believing Edie to be the blackmailer, Paul paid the P.I. to kill her. The P.I. lured Edie to an out-of-the-way location, under the guise that he was interested in buying the property was selling; it was then that the P.I. discovered that Edie had "stolen" the stationary from Martha (with whom Edie was staying while her burnt-down house was being rebuilt).

Since Edie wasn't guilty, the P.I. didn't kill her, but he did report the information back to Paul.

Also it is written in Martha Huber's diary that Mike Delfino had found.