Actually, the ISP that transports your e-mail, any arm of the government, any law enforcement agency and any company representative on either end can look at your mail with impunity.
There are state laws that will govern whether your business has the right to inpect your e-mail.
In many cases e-mail will go through three or more companies on their way to the destination. Once it hits a third party your rights to privacy are null.
If you want to keep it private, encrypt. Be aware, however, that some companies have a policy against allowing encryption programs to be used on company computers. The rational is that you might lock files that belong to the company and not release them when you leave the company. There are ways around that.
Daniel