Not quite sure what the situation is here.... but my first instinct would be to contact the helpdesk for the ISP that the e-mail address is registered with. The risk of course is that they will not be able to help you seeing as you are not the account holder for that e-mail address.
We may be able to provide more helfpul hints but not without more information. What sort of e-mail address is it and how are you accessing it? The numerical code may be going to a secondary address as well, but is there such an address and can you get into that?