| No. E-mail can be made secure, but you have to take a few things into account.
The first thing to understand is that you can’t do much about the addresses, or the subject line. Nothing about these can be made secure. Don’t ever believe them when you read them.
Different systems may allow you to secure the message text of the e-mail, but you have to be very certain what that security is, when it is added, when it is removed, and how you would prove it had been secured afterwards. These are fundamental to you if you are going to rely on the security mechanisms later as proof that something happened.
The easiest way that I know of, is to type your invoices, estimates etc in MS Word then lock the document with a password and send it as an attachment. That way the receiver can read it, print it, but not alter it.
That is how I send work related items to my boss!
If anyone comes across a bullet proof method of locking up an email so it cannot be tampered with, I would be very interested. | 
business email security. 
