This would be a company owned email client. Every previous revision to the email you sent has been saved. Did you change something because you sounded too straight foward? Did you reveal a position that you realized would weaken your arguement? Are you looking for a better lie? Even though you deleted that while writing your email, actually it has all been saved. Do you secretly want your competetors to think they caught something unintentionally sent by you, so they can better act on it?

A simple admin code can reveal your hidden messages. When you the admin/boss, gets the email it comes with a percentage value of how much the email has changed compared to the finished product.

Offer free email accounts to your clients or say that the server only recognizes email of this particular software and then see how well their negotiations stand up to yours.