Your DB is 36 years of age and well able to make contact. You are not his mother.
Your DM has no influence here in what DB does, or what you do. You know you visit her, but you do have a life outside of that too. You don't answer to a soul.
If your mother had DB playing on her mind (understandably) she should have spoken to you about it before going. it is utterly passive aggressive to guilt trip you into taking responsibility for a grown man, who has been well for a decade.
I'd be livid at the email..I know I have my own family issues, which will be triggered but a few things/barbs jumped out on me:
"I would like to have a chat when I'm round tomorrow and thought it would be better to give you time to think about things.
This is saying that you are supposed to be apologetic and be 'told' in advance of tomorrow
Firstly I did not ask you to message DB when I was away but I thought you would.
DM made an assumption, didn't talk to you about it and is now telling you off for it. You are not 12. You are a grown woman with a life of your own and if she wants to run something past you, she needs to say. You are not psychic
I suspect he would never contact you unless things were very bad because he probably would not dare just as I don't phone you because I feel it is an imposition.
WTF is she trying to say here? that you are aggressive, it's justification why she didn't inform you of her concerns AND that you aren't welcoming of her in your life
DB is a good person and you used to seem to like him. He obviously loves DGD like all of us do.
I love all of you and don't want to be an interfering mother ... but I will be anyway I would like people to be a bit closer.
It hurts that you come to X town with DGD to see Fiona but never look near any of us.
Some day I will die and I would be much happier to know that my family felt close to one another. I am writing this now because one day it will be too late.
^ all of the above is just plain old emotional blackmail. How dare she?!^
Please don't reply just speak to me tomorrow when I see you."
now she is telling you NOT to respond, but just allow yourself to be more brow beaten when she comes.