Cox is just screwing with customers in regard to group emails. The do not want you to send emails to groups. The only solution is to go to gmail and use a platform that is user friendly rather than the sham Cox email's system has become.
I have spent hours on the phone with Cox tech support and the only thing I get is the run around. I too have been sending group emails for a non profit thrrough Cox for years, but that has all changed in the summer of 2019. I have learned my lesson and will forgo Cox email system for Gmail.