It is a fact of modern life that you can't unsend email. The problem is that to really unsend email you need a time travel device.
It's a shame, really.
MS-Exchange has the ability to send a request that will hide the email, but most non-Exchange providers don't support the protocol. Besides, the horse has left the barn. You can't put the toothpaste back in the tube.
Gmail has the ability to unsend an email if you sent it in the last 10 seconds. Useful and cute, but not awesome. (Awesomer is their "prove you are sober before sending a message" feature.)
One way to mitigate this risk of wishing you had an "undo" is to send out a first paragraph plus a URL to the entire message. This way you can rewrite, refine, and update the body of the email as much as you want.
We use this technique at work. Suppose we want to tell people that the printing system will be down on Thursday evening so that we can upgrade the print server software. We put the basic message in a 1-paragraph email, and list a link to a document with more info. The link might be to a ticket # that tracks the issue, or a blog post (yes, we have internal blogs), a web page, or a document. We can constantly update the document over time.
Maybe we should extend this. All email should be a subject line plus a URL to the actual message. Made a typo? Correct it. Regretted what you said? Delete it. Called your boss an asshole? Change it to be a loverletter.
You still need to get the subject right, but the message can change. Maybe we could invent a way for the email to be "frozen" once the person reads it (one way would be for the email client to cache the message once it is downloaded). Spammers would have a harder time spamming us, since we'd be able to track their message back to their web site and therefore identifying them would be, well, if not easier, differently harder.
Or maybe we shouldn't even send email. The user interface would still look the same. Behind the scenes it would just be sending URLs.
Usenet made this transition. Usenet was replaced by RSS feeds, which are just lists of URLs. Maybe email should make the same change.