Is there a way to force mail to ask from which account the email needs to be sent?
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!)