Some emails are too important to get wrong. You can dash them off in the heat of the moment, but you should never be too quick to hit the 'send' button. Once they're gone, you can't get them back. Resignation, application, complaint, criticism – whatever that all-important email is, just slow down and think twice before you decide to send it.
Sender's remorse is a horrible feeling. Ask yourself whether what you're communicating is best put down in an email or communicated face-to-face. If you're asking for a pay rise or tendering your resignation, these are probably better done in person so you can explain your reasons to your manager. Similarly, complaining to someone about their behaviour might be best done as a quiet aside in the office, rather than in an email.
People can't always gauge your tone by email, so something said in jest can be taken in completely the wrong way. Slow down, read it over again and ask if there's any chance someone could misinterpret your meaning. Harmless banter to you could be hurtful to someone else. If you've made a joke which might be considered a bit risqué, think very seriously about whether it has a place in your message at all.
If the subject you're emailing about is a particularly emotional one, then it's always worth saving the email to your drafts folder and re-reading it after several hours or even the next day. You need some time away to process it and think about whether you've been completely rational. Things penned at the height of an emotional crisis can often become over-the-top, quite naturally. Once you're in a better frame of mind, you might want to do some rewording.
Finally, check, double-check and triple-check that you've copied the right recipients in. CC-ing in the wrong people is all too easy to do, and the last thing you want is someone reading something they were never meant to have seen.
Don't fall foul of 'sender's remorse'
Ask if this is the best means of communication for the subject
Remember, people can't always gauge tone
If the subject is emotional, take some time away and come back to it
Always check the recipient(s)