cox email rejects pacbell.net email addresses

Can anyone help with this?  My elderly mother who lives in Sun City West AZ switched to cox email.  She can still send email to my pacbell.net email address but cannot receive.  Since I also subscribe to cox, I tried out my cox email box and tried sending a message to it from my pacbell.net email address.  Cox bounced the email back and here is an interesting part of the return message:

Received-SPF: None (protection.outlook.com: pacbell.net does not designate
 permitted sender hosts)

Evidently, cox is proactively rejecting pacbell.net email addresses.  Why is that?  I have had this email address for years and not had a single problem.
Until now of course.

Can someone look into this and get this fixed?
Thanks