I see both perspectives. On the one hand the past is what has everyone in the current situation. On the other, moving forward is the goal so a forward focus makes sense.

We had to do our version of this with my IL clan.  The family multigenerational rotating shit bird seat.  Someone was always in the shit bird seat.  Everyone else would back stab, have back biting conversations, undermine, etc... the shit bird of the moment.   No doubt  my DW and I were regularly in the seat though we never knew it nor would we have cared if we were. My DW finally had enough of the calls about the shit bird from everyone else in the family and put her foot up the asses of the back biters.  

This started an ~2yr cycle that culminated in a family meeting. I was overseas and could not be there in person but I did facilitate the meeting via speaker phone.  I set the rules and enforced the rules of the conversation.  No personal attacks. Attack the problem not the person.  No bringing up past butt hurt incidents, etc.....I then did a high level synopsis of what had brought us to the meeting.   There was some testing of the boundaries, to which I instructed the shit talkers to leave the meeting.  That brought dead silence to which my FIL told his eldest son and wife to shut up and sit down.  BIL1's bovine bride tried to kick BIL2's then SO (later wife and mother of his daughter) out because "She is not family".   I confronted her with "I am the only one who will ask any one to leave and those asked to leave will be people detrimental to the alignment goals the meeting was called for."

We have had about 6 years of comparative calm and pretty decent IL clan relationships since then. Though BIL1's bovine bride will occasionally get lippy.  The new normal is no longer for my DW and I to be the policing authority of IL clan behavior. Now when the bovine bride pulls her shit, everyone smacks her back into her place.

I would consider adopting the future focus that SD has initiated.... but.... hold her to very strict behavioral and interface boundaries as .... conditions of a path forward to a stable relationship with you and your husband.

Seems to be a common problem with step kids, they want to hurl hurtful words at you and twist the past, but when you offer to sit and talk facts they don't want to hear it. Only they are allowed to speak of the "terrible past" even though the adults who were there have accurate facts. My SS was whining to me yesterday about how his mom didn't actually do anything bad... I stopped him and said "repainting the past won't work. You're mom herself admitted to dcfs, the police, and our lawyer all the things she did way back when this happened. There were 3 different agencies that invested her. The adults involved established the facts so we really don't have to go off of your version of events. We've known for years what she did by her own mouth. Changing your story now doesn't mean anything."

he shut up and left me alone after that.... 

moving on in life would be wonderful 

He should tell her that's fine, no discussion of the past, just moving forward - but that means she will no longer send any long hateful emails about all the awful things in the past, either. 

Bratty did this crap during therapy (like what is the point of the therapy appointment if you're just going to send long toxic emails?).

If your DH wants a relationship then he should respond and say 'OK - but moving forward means we discuss any issues.  These emails are not acceptable.  If you can't find words in the moment than write everything out and read it but we're not longer tolerating these long messages about the past'

It really is unnerving when these SKs just toss this crap out into cyberspace whenever they have a brainwave.