Email rejected due to failed authentication checks
User has an @cox.net address. Suddenly having trouble sending to a certain long standing correspondent. When she hits "Sent" is processes it for a few second and sends it to the Outbox. When I look at it a little deeper, there is an error and the SMTP server points us to a webpage of potential errors. Tracking this down, the error is "your email failed authentication checks against your sending domain's SFP, DomainKeys or DKIM policy." I vaguely know that this is a system of verifying email addresses.
My question is, when you have a domain such as @cox.net that you don't manage, how do you get around these sorts of authentication checks on the recipient's side?
Interestingly, when my friend sends a brand new email to this same person, it arrives. The rejection ONLY happens when she replies to one of his emails. I've checked the "reply to" address in the header of a received email, and it's exactly the same as the emails that are successful.