If you are unable to add a new email address due to the warning message "You must enter an email address which ends with @xxx.com", you have set up an email whitelist which will limit who you can add.
To correct this you will need to:
Click on your Name (top right of the screen).
Click on Organisation Settings.
Select General.
Click on Email Domain Whitelist (from the menu on the left of the screen).
Either remove the domain entered or add relevant email domains to the list e.g hotmail.com.