This is not an email address, rather an identifier on what server the user is located on. The chat is based on Matrix and users are connected to different chat servers. As an example a user is located on chat-1, he now has the identifier [email protected], but since this would be annoying to remember and type always you can use the @myteamspeak.com. It will always work.
However just the username is in most cases enough and the @myteamspeak.com should be auto filled by the client.
tl;dr
You only need to tell your friends your myTS username which you can find in the bottom left. It is not case sensitive meaning any capitalization will work.