I sign up with Skype seven years ago. I changed my email multiple times since then. In the end, I found the right email, though.

Remembering the email you used seven years ago is not the hardest thing. It's much worse with the registration month and year. No one remembers that.

Anyway, I submitted both the email and the registration date, but Skype support still doesn't verify me, so I guess it doesn't really matter.