Heated messages usually mix two things: a factual complaint and a tone that makes you want to answer the tone.
Replying to the tone ("there's no need to be rude") keeps the argument going. Replying to the fact tends to cool it down: "You're right that the delivery was late. It ships tomorrow and I'll send tracking by noon."
Keep that reply shorter than the message you got. A long, careful defence reads as more argument, even when every line is polite.
If the facts are disputed, say what you know and what you'll check, with a time. Then check and come back.
There's more on this, including when to step out of the thread entirely, in our calm way to cool down a heated email thread.
Grais · m@grais.ai