It is well known that email encourages people to write incendiary or insulting or idiotic messages, expressing sentiments they would never express if they thought a second time. If you receive such email, put it aside. If the email has been broadcast, surely put it aside.
Most of the time, you will want to save the message, never respond, and if asked if you read it say it must have been lost in the Inbox and have them send it again. They won't.
If you feel compelled to respond, think twice. And in any case, never respond immediately--wait a week. But if in the end you respond, respond in an unemotional manner, stick to facts, never argue. It won't stop the flamer but it may help.
I have found that this sort of email comes out of insecurity, anxiety, and an inflated sense of importance. There is little you can do about those sources. If things continue, you hope the source will self-immolate, so to speak.
I know that I have an earlier post on this topic.