1. Make sure you are using the EXTERNAL\ before your username, for example: EXTERNAL\JOHNSMABCD
2. Make sure you type the Username and password, do not copy and paste for the email.
3. Check to see that when you typed in external\username and password that the domain changed to EXTERNAL
Note: If your company uses Sharepoint it may give you the option to enter password or login as a different user. At this point select to login as a different user so you can enter in your EXTERNAL\username and password.