I've had way, way worse emails than that (and your one is wholly unacceptable too!). Current line manager is regularly "disappointed", say if I haven't immediately fulfilled one of her demands (eg. "I need your annual leave by x" when I've just returned from absence due to stress at work, and everyone else has already booked theirs so it doesn't matter one jot when I am off! or, "we agreed that you would submit y by z date, and I'm disappointed you haven't" - when I have agreed no such thing! I've had even worse from her manager - and been told I'm "insubordinate and challenging" when I've queried a practice that's irrational and unreasonable! I asked whether she wouldn't challenge her manager if she disagreed with them - and she said no!!
Saddest thing for me is that I am massively more academically and professionally qualified than either of them. Neither has even a degree never mind professional qualification. I'm educated to masters level in my field, and have also several management qualifications.
Problem? I'm too old.... nearly 60 and therefore invisible. I've been passed over for promotion in favour of someone half my age, with a small fraction of my qualifications. I can't really leave now either, for the same reason.
Soooo I just have to try to find a way not to care... and that's not me.
If you can get out of this situation OP - you should. It's not going to change.
Guess where I work folks... HR... Employee Relations to be specific. I'd never advise anyone to pursue a grievance or dignity at work complaint. The more senior person is always going to be supported by senior management because they are regarded as more valuable, even if they're toxic. I don't agree that HR is a management stooge though. Making sure that people are treated fairly means that the organisation stays out of industrial tribunals etc. I can honestly say, I've always dealt with cases/complaints as fairly as I can, and if the manager is in the wrong, I will say so.
Good HR should be "somewhere between managers and management". I came across this quote when researching my masters and it has always resonated with me.