Identifying email account in the Inbox

I'm on an old Mac Pro, and using 10.14.6. I have three email accounts, and so far I've been getting input for two of them, as expected, in my inbox, but not differentiated as to account. They all wear the identifier of the oldest account, in my case Earthlink, even if they came via my iCloud account. I would like to keep track of the account being used to see, at a glance, which accounts give me problems, so I would like to see the email origin domain name in the Inbox. As it is now, every email is converted to an Earthlink origin. How can I change that?