Emailing is like sending a letter in ordinary post card. It has to go all the way to reach the recipient. If they wish, any webmail admin can access someone’s mails. Okay, lets assume that you have a dedicated server or VPS so that no one can read the mail going through your server. But what about the recipient? His email provider can certainly read it. These things just work on belief.
If you want that level of secure connection from your end, buy SSLs, if possible, get a dedicated or VPS.