Unused Apple ID now shows up in Mail accounts

I created the username jdj74 specifically for use here in Apple Discussion forums. All of a sudden, jdj74@mac.com and jdj74@me.com show up as options from which to send emails in Mail, as well as my iPhone, though they're not listed as accounts in Mail or the phone or iCloud.

FWIW, my primary email account and Apple ID is another @mac.com address. And that account is the email associated with the jdj74 Apple ID.

Any ideas?