Apple mail
How do I access my Mac.com email?
A receipt was sent there and I didn't even know I had this email account.
iPhone X, iOS 13
Your iCloud email address is the same as the @mac.com email. Just log into your iCloud email at: https://www.icloud.com/mail/
Here is a support article with more information: https://support.apple.com/en-us/HT201771