Do not engage, instead observe.
Relax your tongue because only a tensed tongue speaks angry words or engages with anger. When you start by relaxing your tongue and making it broad so that it fills the entire plate of the mouth, not only do you become more grounded in your body, but you begin to relax your whole body as well.
Focus on your breath to remain calm. Observe how what they are saying and doing feels in your body. Identify those thoughts and feelings with one word: Thought or Feeling. Identify those with one word: Positive, Not Positive, or Neutral. Break it down further with one word if you have to and then another if still needed.
Example: Feeling, not pleasant, anger, defense, tense. or Thinking, not pleasant, irritated.
Dissolve all thoughts and feeling by validating them with one word…, while keeping at least 25% of your focus on your breath and 25% on keeping your tongue relaxed and 25% of your focus on your thoughts and feelings and 25% of your attention on the person freaking out on you.
Once you are grounded, figure out what they are actually saying by turning it around. When you understand, you will be more understanding. Perhaps they are saying they hate you because of this or that they are accusing you of doing. Turn it around and realize they are saying they hate themselves for doing exactly those things...
If you are not able to bring sanity to the situation with insightful words, then say something like, “I understand you’re upset. When you are feeling more calm, I’ll be more than pleased to discuss this with you so we can resolve the issue.”
However, if they’re simply insane and there is no possibility of reasoning with them because maybe you didn’t read an email or something like that you never received, or just haven’t gotten to it yet, perhaps agree by saying something like, “I’m really sorry you’re upset. When I figure out how to make it better, I will.”
Then make it better by leaving and never return.
Remember you cannot reason with the unreasonable and the insane are not prone to sanity. Accept it and don’t take it personally and do not engage in it. Keep your communications respectful, but do not engage in defending yourself of whatever insane thing you are being accused of.
After you realize the individual is insane, don’t subject yourself to them again. Instead respectfully avoid them even if they happen to be a in good mood that day.