Why does Outlook keep prompting me for my username and password?

This will happen when either your username or password in your account settings is incorrect. 

• First make sure you are using your full email address as the username (ie username@YourDomain.com)
• Make sure the username is not an email alias, but the actual user for that account
• Check over the password, and if it is not working you can reset it in the Plesk control panel