When you host your domain as GitHub page, GitHub doesn’t give you option to host your domain email.

Which options you have:

1. If you bought you name on godaddy you can host email there (but only one).
2. Godaddy allows you to forward 100 email

3. If you have Google Apps account, you can host email there

   Unfortunately Google doesn’t allow subscription to free tier of Google Apps, but if you subscribed before you can keep it.

4. You can use other services, for example Mailgun or Zoho, see article Custom Email Domain via Gmail when GitHub Pages is Your Web Host