If your job requires emailing at all, I've also picked up a few methods for responding to rude emails.
In my current job I strictly work B2B (business to business) and before this job had only worked retail and food service. I thought when I broke into office work that I was finally done with, well, the types of things you deal with in public facing customer service roles (the constant rudeness, dehumanization, unnecessary hostility, etc you know) but unfortunately after nearly two years sending and receiving emails in a B2B setting I regret to inform you that people are still rude dehumanizing jerks even when you are both professionals at work representing your own respective companies.
So, here's a few firm yet professional replies I've picked up.
If you are emailing with a supplier (as in a situation where *you* are the client) obviously you still need them to function at your job so you still need to be professional and remain on good terms with them, but unfortunately with the way things work when you are the client you do get a little more leeway in being firm and standing up for yourself. My go-to response when a supplier is being out of line rude / unprofessional / disrespectful is:
"Unfortunately I have to express disappointment at the lack of professional tone in your email."
Every time I've used this with a supplier I get an apology and a change of tone from thereon. You don't want to escalate the aggressive tone in a work related email, that never ends well, once they drag you down to their level there's no coming back from the fact that you just normalized the aggressive and unprofessional tone of your email communications with this supplier, so you want to keep the "high ground" so to speak. When you keep on the high ground instead you can shift the tone back to professional and courteous, which obviously makes your job easier.
However, you're still not letting them off the hook or just letting it slide with expressing your disappointment, and it's also good to be (professionally) putting your foot down and calling out the behavior so they don't get the idea that they can throw tantrums at you again in the future in a professional setting.
Handling rude and unprofessional emails from clients is obviously more tricky, because depending on what industry you're in the power dynamics are typically shifted more in their favor, so you have to watch your tone a little more.
I'm lucky enough that at my job I'm not only part of a union, but I have a boss who takes my side when clients are being unreasonable and unprofessional, so I can get away with a:
"Let's keep the tone of our communications professional, shall we?"
It still has a bit of a "customer service" tone to it, it still gives you the high ground (again, you don't want to sink down to their level and normalize rude / petty / hostile interactions with this client, it will make your job harder and more stressful if you do) but it does still address the behavior and let them know you're not just letting it slide.
