For those that haven't read How to Win Friends and Influence People, Lincoln used to write long letters telling his generals how incompetent they are, but never sent them. It was his way of getting it out of his system.
I find it best to make sure not to fill out the "to" field in emails which I know are going to be "controversial".
An errant tab+enter in gmail can really make the day a lot more complex.
Edit:
Once it's been toned down, if it still needs to be sent (usually it doesn't), adding the recipient is the last step, and final filter that a nasty message should pass.
Reading a fuming email to a spouse or friend before sending is also a good way to keep them in the drafts folder and out of other people's inboxes.
I've written many letters I've never sent. I prefer communicating in writing, anyway. And they are not all venting or letters of frustration. Some are just my way of formulating an idea, opinion or position on a subject. In addition to "getting it out," it helps me to better organize my thoughts, rather than just having them swirling in my head. I can then have some confidence that I have fully extrapolated my thoughts and provided justified for myself the way I feel.
Writing longhand helps me blow off steam. I'll write long, scathing diatribes full of bad grammar, misspelled words, and rude doodles. Then when I'm feeling calmer, I'll drop them in the shred bin.
Some people recommend doing this with emails as well - type but don't send. If you do do this though, make sure you don't send the angry version of your email by mistake: http://www.dailymail.co.uk/news/article-2951973/Is-rudest-re...