recover old email address.

Hello. As of early 2025, all cox.net email addresses transitioned over to Yahoo. You can find more information about this here: https://www.cox.com/residential/support/coxnet-email-transitioning-to-yahoo.html
-Allan

An Internet search for "what happens if a cox email account was not accessed after the transition to yahoo mail" resulted in the following:

If a cox email account is not accessed after the transition to yahoo mail, the account will remain accessible for a limited time. Once the transition is complete, the email account will be supported by yahoo and accessible even if the service with cox is discontinued. However, if the account is not accessed for an extended period, it may be subject to deletion or other actions taken by yahoo. It is advisable to regularly check the account and ensure it is active to avoid any potential loss of access.

Your old Cox email account was probably deleted by Yahoo due to inactivity.

Edit: The following is Yahoo policy.

https://help.yahoo.com/kb/account#/ 

Yahoo policy > Reasons Yahoo deactivates an account

Depending on your account's history, it's possible for it to be removed from Yahoo and become inaccessible. You can always sign up for a new Yahoo Account, but keep in mind that your old Yahoo ID won't be available for you to use. These actions can result in an account being deleted:

• You requested that your account be deleted.
• You haven't signed into your account within the last 12 months.
• Your account was used in a way that violates our Terms of Service.