Standard email messages are simple text files. They can be read by anyone. The employees of the ISP have full access to your email while it is waiting to be sent. Potentially they could read it, modify it and take copies of it.
Most companies use outside consultants or service companies to maintain their networks.
Email is often routed through the outside computers on its way to the Internet. As with ISPs, employees of these service companies have full access to the contents of your email. Routers Internet data flows around the country and the world from one node to the next. Administrators at these sites could capture copies of the data that flows through their routing equipment. In short, sending an email message is like sending a postcard.