New iPhone: me.com email login error

Hi

I have just upgraded from IPhone 12 Pro Max to Air.

I’ve had a me.com email for years and it’s my main email for work and professional contacts.

This is NOT my primary on apple account.

I have signed into all my other emails ok but the me.com says the account is mine or the password is incorrect. It’s not.

So this is something to do with it being a me.com address and the way my accounts are situated but I don’t understand.

I know MobileMe was shut down but I’ve always been able to change my password and use the email address.

Right now I can’t add this important address to my new phone.

I’ve read other threads about this being my iCloud address but that isn’t working as they’re different and I don’t receive my me.com mails through that account.

There’s something I don’t understand here.

Its as if the account exists and recognises it but I don’t have a Mail account in Mail app