Hi celynnen,
welcome to the Apple (user-to-user) Community.
Am no expert in this area, yet your request deserves some response:
Wondering if Apple support's online articles may be helpful:
Eg: iCloud: About your @icloud.com, @me.com, and @mac.com email addresses - Apple Support
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Re:"it comes back as an address that does not exist" : "is there someway to reinstate my old email address?"
Seems your Apple ID @mac.com address likely can still be used ...just iCloud & Mail may not be turned on
As you already have an @mac.com email address, (NB: article does not mention earlier addresses )
see if you want to: Set up iCloud Mail on all your devices - Apple Support
On another tack:
If you no longer want to use the email address that's associated with your Apple ID, you can change it.
You won’t lose access to your contacts, purchases, or other account information
How to change your Apple ID to a different email address:
Change your Apple ID - Apple Support
All the best :-)
