Without access or information about your email interface and access method we can't help you.
Note I'm not asking for real creds or anything here, but your post basically boils down to. I'm trying to do something like this and it doesn't work, how do I fix it. Based on that level of detail all we can possibly say is "do it properly"......
0) What are the actual errors hydra is giving you.
1) Can you actually access your email account via HTTP?
2) Is index.cgi the actual login page? Are the POST parameters you outlined above actually correct? Did you view the source of the page and do you understand it?
3) Have you tried to manually build the request with known creds and submit it? (use curl, or burp, or something like that)