I've just heard that Gmail has undo for sending e-mail. The way it works is that it delays sending the e-mail by, say, a minute, and during that time you can cancel the sending.
This would totally rock in Evolution. You see, Evolution has two key combinations that are unfortunate in their proximity: control-backspace deletes a word, and control-enter sends the e-mail.
Anyone who has written e-mail while angry knows the feeling of deleting words furiously. Consider what will happen if you're deleting an angry, half-written flame with control-backspace, and press control-enter instead...
Undoing sending an e-mail would allow you to recover from that.