mailer-daemon@comcast.net when sending to EarthLink email address

[Rvalv]

Have sent emails via iPhone to my sister with no problems until last week she has EarthLink. Net email address. I also tested my RMI.Net which is part of EarthLink service and getting same mailer-daemon@comcast.net error.
How can I get this fixed

[Bmones]

I have the same issue but only when CCing my own EarthLink address or sending to another EarthLink address. My service is Comcast and the message is " mailer-daemon@comcast.net " and shows as a permanent failure.