Fast Company has a good article detailing four items to exclude from your work emails. This is work focused, but the idea behind excluding these is relevant to personal emails as well.
These are good rules of thumb. This is largely about retaining your humanity in the bustle of work life. We need to see the people we’re communicating with as individuals, not communications endpoints. This is sometimes important individually, always important in the aggregate.
I sometimes send or respond to 50+ emails a day as tech lead of a ~15 person agile team, with a number of core members and an equal number of temporary staff, with all the logistical and technical setup, expectations setting, and insecurity that goes with that. It’s more than a handful, and the only way to keep the team productive and keep everyone engaged and reasonably happy with their work is to treat them individually as human beings, not just collectively as a team.
For a long time, I had a sticky note on one of my monitors reminding me to “Slow Down. Be Kind.” I brought the note home during an office move. I need to put the original back up at work and make a copy for home. I’m often better at following the note at work than home and I need to fix that.