I registered a Minecraft account before September 2012 and did not migrate to a Mojang account. The original email used seems to be long lost. In the migration portal (https://account.mojang.com/migrate), when clicking on "Forgot your email", the websites directs me:

"If you no longer remember which e-mail you used to create your Minecraft account you need to visit our support center in order to get help with migrating."

The support center link, under the heading "Forgot your registered email address?" instructs me to "Locate your transaction ID", which requires me to look into the email that was used to sign up. The email, of course, is long lost, which is the reason why someone would want to recover the registered email address in the first place. 

I think the solution would be to allow Minecraft account web sign-ins where the user can see their email address, or to allow users to see their account email address within the launcher.