Usually a dream during bereavement is intended (by the dream director, who is a version of Higher Power, aka God) to help direct the grieving person through the process to eventually get the important benefits from the relationship that has come to an end on the earth plane--though the inner relationship will continue . The keys to understanding what is NEW NEWS about your dream are to be discovered by comparing what you know in waking awareness about the subject(s) of your dream with what the dream seems to be saying/showing, given that much of that message is in a symbolic language.\So did each individual envelope have your name on it? or did you "just know" that all the envelopes had your name on them? or did the Hole from which they emerged have your name on it? [All these apparently inconsequential differences could hint at specific meanings, because the dream director doesn't put random elements into dreams; everything makes sense but ONLY throuigh discovering the perspective the DREAMER has via HER REACTIONS.
Your mother was NOT SMILING: How would you expect the meaning of the dream to be DIFFERENT if she WERE SMILING? [I'd guess "she'd be showing you that she's happy--after passing--in the hereafter. So WHAT WOULD YOU GUESS?]
"She was very firm" suggests to me that "she was very businesslike." Is that the right word for you? (I'm trying to get you to use your imagination on assessing other possible dream scenes that didn't happen, so I can find out thru YOUR imaginal mind what the actual scenes DO manifest as products of your unconscious mind.
And one more question for initial exploration of the dream: What recent event(s) or thought(s) might be related to the "letters" in your dream--tyhat is, WHY DOES THIS DREAM COME TO YOU NOW?
I forgot to ask about one other specific element: How would the dream be different if she were walking in YOUR back yard instead of her MOther's back yard? And you didn't answer my questions about her NOT smiling (I guess YOU're assuming she's not smiling because YOU're not happy with your husband. Why WOULD YOU NEED TO GET SO MANY ENVELOPES? Is there an avalanche of Unfinished Business coming out of your maternal grandmother's backyard--which might mean you have a LONG TRADITION of unfinished business from your maternal inheritance-line, which your mother is quite "firm" about: Why FIRM? By not offering words from your imagination, you make me GUESS from my imagination, and my source of unconscious insight into your dream is WAY LESS VALUABLE than YOURS.
What is the Endless Stream of Maternal-Tradition Envelopes for YOU TELLING YOU?
That's possibly the key NEW NEWS of this dream. (but you'll have to use your imagination to begin reaching for it!)
Were the letters obvious Business Letters? Or do you have snarlups with your mother's will or other property issues?