Old email address for Apple ID keeps popping up!

I have a number of Apple Devices and one Apple ID account as far as I am concerned.

Linked to it are an iPhone 5, iPad, and iMac x 2.

I changed my email address which was my account name. I logged in and changed / confirmed about 3 years ago. SInce then everything works OK.

I can successfully log into the iCloud etc using this 'new' email address.

But, when starting Macs up, I keep getting a pop up box saying log into account from the 'old email address'

Where can I access this old version and kill it off without fear of deleting the new one?

I went to the icloud to check settings, and only the new email is shown as you would expect. I tinkered with a postal address, and again, the old email popped up as the account to use password for to make change. Grrrrrr.