Need help to access my .mac email.

I am using 11.3.1 on my iPad model MF004LL/A. I am trying to access my .mac email account. I would also like any information for accessing on the iCloud site and/or my MacBook Pro which is version 10.13.4, 2.7 ghz.

The error message says "Address not found. Your message wasn't delivered to [personal]@mac.com because the address couldn't be found, or is unable to receive mail." The response from the remote server was " 550 5.1.1 unknown or illegal alias [personal]@mac.com

I have looked on the support information but cannot find an answer that works. I had a chat with a support person and was disconnected. I just want to use my .mac email as my main email and want to access it any way I can.

Please help!