I have multiple user accounts under mail and more often than I'd like I send an email from the wrong account. I know it is possible to put a default account, but I would like a way to make me sure I am checking the account I am using before I can send the email, like leaving it "blank". Is there an app around doing something similar to that?
I would add this as a comment instead of an answer if I had enough reputation.
An untested dirty solution might be to add another mail account with incorrect login info (so you can't actually send mail from it), and set this account to be the default account for sending new messages (see NEO's comment).
This way, if you forget to change the sending account, it will fail to send (it'll get stuck in your Outbox, I believe; obviously not the ideal solution if you don't think you would notice this!)