Add me to the list; I suspect my mother has an undiagnosed personality disorder. I had cut her out of my life. Now I'm her guardian (the "guardian of the person," as the legal wording goes); she was declared incompetent last year. I do this out of a sense of duty, not love. If I weren't her guardian, she'd have been turned over to the state, because my brother, who has been dealing with her for years, refused to be named guardian. Yes, I had a choice; I could have let the state take responsibility for her.

Anyway...

BMC, you get to decide how you want to deal with your mother, what the boundaries are. You cannot change her, but you can change how you respond to her. (I'm sure you know that already; just reminding you!) If you don't like the idea of accepting a certified letter, don't accept it.

I like the idea outlined by another re: not retrieving the letter, asking your mother to ask you the question directly before you open the remailed (by regular mail) letter, and letting her know you will answer within [name your time period here] IF you are actually open to answering her questions. If you are not, I am sure you have your reasons for not wanting to do so, so let her know that... once. That's the boundary. When she crosses it, shut down the conversation immediately. Acknowledge any future letters you receive when you talk with her ("Got your letter, Mom, but knew I'd be talking to you, so didn't open it; you know how I feel about the letters. How about those Red Sox?"), but do NOT talk further about the letters other than to acknowledge receipt and restate the boundary.

I also like the idea of examining why the letters upset you, not because I think you owe your mother that, but rather, because you owe yourself that.