550 Administrative prohibition - unable to validate recipient Did you receive the above error when sending an email? 5xx error are permanent failures. "unable to validate recipient" sounds to me like it's the same as "user unknown" Make sure that you correctly typed the email address that you are trying to send to. Also double check to make sure you don't have a bad address book entry to and leading or trailing characters such as spaces surrounding the address. You may need to pick up the phone and call the contact to find out if their email address has changed.