is it work email or personal email?
regardless, the email server *should* be using a secure connection. that is, if you look at the URL here on cruise critic, it's https, not http. the s implies there is a secure connection between your device and the server, so no one can see the contents of the email from your device to your email server.
THere is nothing "extra" to buy for extra security except perhaps a VPN, but all a VPN gets you is an extra layer from you to the first email server.
So note i say first email server, becuase... emails need to be sent server to server and that is not necessarily guaranteed to be secure (and you have no control over that, but for exampe in Gmail you can see if an email was sent end to end secrely or not).
So with https without vpn, what can you see? well, just what url you're hitting (e.g. boards.cruisecritic.com), not the contents of what was sent.
That being said, https is secure. And theoretically so is the email (if configured properly)