Hide the Email Address and Icon
The best way to do this is to by simply configuring Divi not to display an email address in the first place. You can do so by going to "Divi > Theme Customizer > Header & Navigation > Header Elements", deleting anything in the "Email" field and publishing the changes.
If you need to do it via CSS, you can hide both the email address and icon with this:
Related Post: Adding CSS to the Divi Theme
Hide the Email Icon
If you can see the email icon, but no email address, the first thing to check is that there aren't any stray spaces in the email address box – as the email icon will show even if there is just a single space in there. Deleting the spaces should get rid of the icon.
If that doesn't help, or you actually have an email address you want displayed, you can hide just the email icon using this CSS:
Hide the Email Address
To hide the email address, but leave the email icon visible (and clickable), you can use the following CSS:
Once installed, go to "Divi > Theme Customizer > Divi Booster > Social Media Icons". There, click "Add a Network" and set the "Icon" to "Mail" and "Links to" to "mailto:[email protected]". This will add an email icon alongside the other social media icons in the top header and/or footer.