Since the error is authentication failure so plesae verify the credential given in Email Account rule is correct or not. You can do this by Clicking on Test Connectivity button on Email Account.
If connectivity is not an issue then check if you have multiple Email Accounts defined in PRPC ? if yes then by default PRPC picks the email account which matches the current work pool name or else the Email acount named 'Default' is picked for sending correspondence. Please see which email account is used in your case.