If an account has been migrated you can't crack it by using it's username to log in...
To find out their email address you go to https://account.mojang.com/migrate and log on and it will show it to you, simple as that.
Good luck cracking!
I have been cracking for a while with egg.crack(); and I have always wondered how to find somebodies email with just their username and password. Mostly because lately I have their password but I can't do anything if their migrated. It really sucks when they have a good username and I can't use it. If anybody could help that would be awesome and I will reward you with 10 accounts I cracked.
If an account has been migrated you can't crack it by using it's username to log in...
To find out their email address you go to https://account.mojang.com/migrate and log on and it will show it to you, simple as that.
Good luck cracking!