Emotional Firewall: Never let your immediate emotions dictate your permanent written record.

Write the angry email. Put all your frustration into it. Then, remove the recipient's email address and save it to drafts. Do not look at it again until the next morning.

Next time you are triggered by a client, write the draft, sleep on it, and rewrite it at 9:00 AM. You will always delete the venom and keep the boundary firm.