You talking to me?
Have you ever sent an e-mail to the wrong person? It can be embarrassing at best and a disaster at worse. Check the intended recipients before you press send. For a long and potentially contentious reply (and see other advice below), consider removing the names and reinserting them when you are ready and happy with the content.
Stringing them along
Long e-mail strings can be dangerous. Recipients can change and content be added of which you may be unaware. Consider starting a new chain or at least ensure that you check the entire string that you are forwarding before sending it on.
An e-mail is not a chat over the coffee machine or the place for a hot-headed response. It is a permanent publication that can give rise to a claim in defamation by a third party. Check your content to avoid threats or action.
At the click of a mouse an e-mail containing private and confidential information can be forwarded without your authorisation or control to many and it can happen quickly. Don't start the leak, unintended or otherwise; it could lead to an all out flood.
Count to ten
Count to ten before you send. Make time if possible to come back to a potentially contentious e-mail after a moment or two of reflection. Imagine your e-mail being read out in court and ask yourself, is that what I want to say and how I want to say it?