Never send it angry. Twenty minutes is usually enough.
Not an hour. Not "sleep on it", which you won't do. Twenty minutes, and then read it again.
What changes in twenty minutes
The line you were most pleased with stops looking clever. That's the one that would have been read out.
Log it in the meantime
Write down what was sent to you and how it landed. That's the record.
Then decide whether a reply is needed at all. Often it isn't.
Remember what a provocation is for
Some messages are built to produce a reaction on the record. Recognising that as it happens makes it much easier not to supply one.
Your hostility is evidence too
Exactly as usable as theirs.
The messages that damage people's cases are almost never the ones they sent after thinking about it. They're the ones sent at 11pm, fast, in reply to something designed to get exactly that response.
Why twenty minutes
Because it's short enough that you'll actually do it. That's the whole reason for choosing that number over better advice you'd ignore.
When the twenty minutes are up, if a reply is needed, keep it brief, informative, firm and friendly: the practical part of their message answered, and nothing else.
General information, not legal advice. Court process differs between countries and regions; check anything that matters to your case with a qualified lawyer where you live.
Chapter 18 of the workbook covers communicating on the record. Worksheet M is a one-page draft sheet for difficult replies, with a final read-aloud check before anything is sent.
See the workbook & spreadsheet