Why not just click only on the account for the company you're working for at the moment, instead of the union of all of them? Make sure you have the "Send new messages from:" field in the Composing preferences set to "Account of selected mailbox" and your outgoing email will come from that account only unless you deliberately change it while composing a message. Additional bonus: you don't see all the mail relating to stuff you aren't working on for this employer!
