Elderly neighbours can be the worst. And in particular, if they are homeowners, I've personally experienced a lot of resentment from elderly residents towards renting neighbours. As someone up-thread said, it can be a combination of too much time on their hands, and with age, sometimes, can come finicky-ness. This extends to not only their space but the space of those around them.
I'm talking very generally here about these sorts of problems in particular, not entire sweeping statements about a generation, as most normal people of any generation do not behave like this.
I'm really sorry your daughter is being bullied by the residential clique and bombarded with letters, comments, and moaning so soon after moving in. If the complaints were genuine, you'd think those who have an issue would have enough self-awareness to understand that that's not the way to go about it towards a brand new neighbour. Sadly it sounds as if they have the self-awareness of a rock, judging by how they pushed the last neighbours out, which is important to consider here.
Your daughter is entitled to smoke anywhere she wants, on the outside of her home. That other people dislike it is neither here nor there. I'm sure there are plenty of annoying habits they have (like bullying renting neighbours) that I'm sure she'd prefer they didn't impact her with - I guess it's swings and roundabouts, isn't it?
I would advise her to wear headphones when she goes outside for a while, whether that's to smoke, potter about on her boundary, or whatever. Completely grey rock and ignore them as you have done, do not even respond. Do not acknowledge you have heard them. If they can't ever be pleasant, they lose the right to be heard, in my view. Keep the letters for future reference, she may well need them, to show the police, if, as I sadly suspect, the harassment doesn't stop.
Had the neighbours been welcoming, then broached an issue or so, in an appropriate manner, I'd advise her to listen to them and try to be accommodating even if they are a little unreasonable, for the sake of keeping good relations, but there are already no good relations to keep due to them being utter arseholes, to what I gather, is a single female who has just moved into her new home. They have form, and will never be happy with any renter there.