Reactivate a dormant @mac.com account
I created a @mac.com email address 12 years ago when I made my first two or three iTunes purchases. After that I switched to my usual email address.
I have just transferred all my files from my 2012 iMac to my new macBook Pro. iTunes informs me that to authorise these first few purchases on iTunes, I have to sign in to my @mac.com account. I have never used it for email purposes and have no idea what the password could be. I've made attempts and have now blocked the account:
"This Apple ID has been locked for security reasons.
I'm invited to reset my password, but I have no way of receiving it since I don't use the email address. How do I get access to this account?
Many thanks for your help!
MacBook Pro, OS X El Capitan (10.11.3)