Can't send email to other Cox accounts

I cannot send emails to other Cox email accounts from my IMac or MACbook Pro.   I can send and receive to anyone, but my emails to other Cox email accounts gives me the following message:

Cannot send message using the server Cox

Verify that you have addressed this message correctly. Check your SMTP server settings in Mail preferences and verify any advanced settings with your system administrator.The server response was: <xxxx@cox.net> recipient rejected

Select a different outgoing mail server from the list below or click Try Later to leave the message in your Outbox until it can be sent.

Sending from: Me   aaaaa@cox.net

I tried sending an email to multiple recipients and got this message. When I removed the recipients with cox.net addresses the email went out.  

I confirmed the correct server settings. I can send a email to myself and I receive it.