I can not make changes to email account settings in Mail.

Basically, current server has changed to yahoo server, but I can still use cox.net address. I have the Incoming and Outgoing information to change. I tried to add my @cox.net address to Yahoo. I followed the instructions and I'm able to see both of my inboxes for my two emails, but only through the Yahoo mail site. I still have my yahoo email on my iMac on Mail, but I can not duplicate the the old address onto yahoo through iMac mail.