Do you mean iCloud Mail? I assume it is as secure as all the other iCloud services themselves. Personally, I don't use iCloud (except for the "Find my" feature). But you can set up any third party Email service with the Apple's Mail app. I use Mailbox.org as email provider and have it set up with the Mail app on my iPhone. It works flawlessly. Third party email providers often come with additional costs (the lowest tier with Mailbox.org was 12 € each year when I switched from a freemail provider a couple of years ago). I don't know if iCloud Mail is free of charge, other than additional storage space.