Thunderbird and GMail both now provide a fairly intelligent feature that catches when you say “attached” or similar and will stop you if you try to send the message without an attachment. (And it’s not too annoying if you said “attached” or “attachment” without meaning to attach anything: “The cable is attached to the rotor” doesn’t mean that I’m attaching a cable to my email.)
One thing I really want, though, is the ability to ‘lock’ an email so I don’t accidentally send it. There’s some keyboard combination that means “Immediately send this email,” and, on several instances, have somehow bumped it in the middle of sending an email. (For what it’s worth, I’ve done this on every mail client I’ve ever used, so it’s not specific to one email.) I’d love to be able to click a little lock that would lock out the email from sending until I clicked it again.
The way I normally get around this is to fill in the “To:” field last. (If I made an email client, I’d move it to the bottom.) This doesn’t work for replies, though. If I’m replying to one person and have a long email, I’ll pull out their email address and re-add it when I’m ready to done. But when you’re doing Reply-All, it’s not practical to do this. And Reply-All to a sizable list is exactly when you want to be very careful that the email isn’t sent unless you explicitly mean to.
The other option is to write the body in a text editor and paste it in when you’re done and it’s proofread, but I think this is fundamentally broken.
(If you take nothing else away from this, though, take the tip I picked up about making the To: field be the last one you fill in.)