KF5 system settings doesn't have an email address setting any more, so these options should be removed from KAlarm.
It is still possible to set a user's default email address in System Settings, in Account Details -> User Manager. However, the a much more likely place to set it is using the KMail wizard. The option wordings and descriptions should be revised accordingly.
Text has been fixed for KDE Applications 17.04.1, commit b3692451877fa8097841fe426b6bf193f7af17de.