You are using Email to Case Premium and have received an error routing email similar to the following:
A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed: email@example.com
(Undelivered): 550 Address owner is inactive: firstname.lastname@example.org
Emails processed via Email to Case Premium (E2CP) will create / update cases via a specific Context User. This user will display in the Created By field for all cases created / updated via email.
The above-mentioned error indicates that your E2CP Context User is currently inactive.
To update your Context User to an active user, navigate to Setup > Develop > Email Services > Email to Case Premium. Each of your E2CP routing addresses should be listed at the bottom of the page, with the Context User displayed to the right. To edit the Context User for the addresses, click the Edit button to the left of each routing address.
To reactivate your existing Context User, follow the steps above to locate who your Context User is, and then navigate to Setup > Manage Users > Users. Next to the user record for the Context User, click edit and locate the Active checkbox. Ensure this checkbox is populated, and click Save to reactivate the user.
In general, we recommend using a dedicated API user account if you have one to prevent this issue from ever occurring.