Unbind account with email address

When I first started the game, I played on pc and linked my character to my first email address.
I then played on mobile and linked another character to my second email address.
Later I found out I could play my pc account on mobile, so I just logged in with my pc account and didn’t save my mobile account detail.
Now I want to start a new character and link to my second email address. But apparently I can’t because my second email address is taken by that short-lived character. How can I unlink my second email address without account detail?