Apple has begun sending out e-mails to existing @mac.com and @me.com e-mail users to notify them that an @icloud.com account has been reserved with their established username.
"All new iCloud Mail accounts now come with an @icloud.com email address," Apple's note reads. "As an existing user, we'd like to offer you this new address as well."
Users are free to continue using their current e-mail address as well, if they do not wish to switch to iCloud.com. No matter which address users use, they will continue to receive all their e-mail at me.com, mac.com and icloud.com.