I've often seen folks email me (or a mailing list) with an email that trails off, unfinished, and later they write another note apologizing that they hit send by mistake, and the mail program sent it off immediately. You don't need to put up with that behavior, if you wish it didn't happen while you were writing email. In Outlook at least (and I'm sure in most mail programs), there's an option that I always turn off to stop it sending email automatically. Instead, the mail goes out when it otherwise collects email (another option whose time you can control).
In Outlook, the option to stop it sending automatically is in Tools>Options, and there choose the "mail setup" tab. On that tab, under the "send/receive" section, is "send immediately when connected". Uncheck that, and now mail will remain in the outbox until you the automatic collection of email (or if you manually send it with tools>send/receive). The only negative I can see to this is if you've come to expect that when you send a note, it goes out immediately (such as if you're participating on a list and your urgent reply is important). Again, though, you can always just manually send in that case (as I do) using that Tools>Send/Receive option.
Hope that helps someone.