I've recently heard about some people who used an invalid email (an email without @ or . symbols) to sign up for Minecraft back in the day (so now it's an unmigrated account). According to my friend, if you try to migrate his account with the email "jtblasi" (which is correct), it won't allow it.

Now before you close this ticket and shove us over to support:

I think the Mojang site should check for accounts that exist on the email the user inputs in the "Migrate account" form before disallowing it to be migrated because of an invalid email.

I've heard lots of people have their Minecraft accounts on bad emails and now it's impossible to migrate, unless you have proof of purchase you can give to Support, which is unlikely because you used an invalid email to sign up.