Some special characters are not supported in user email addresses, such as: ' ´ ` ^ ¨ / * &
Characters like these are also not supported in various mailing applications, such as Gmail or Outlook.
If you need to add a user which contains some of them, you may ask for an alternative address without the special character/s or create a temporary email address or a fake logon, and provide the logon and password manually to the user.
You can check more info at: Characters allowed in user emails