We are working on a web project that has a password reset feature.
Now the problem is, between "Password Reset Mail" and "Reset Password Mail", which is better?
It depends where that text belongs. If you have a button on a webpage to reset the password, it should simply say “Reset password by mail” or, shorter, “Mail password reset”. The mail itself would be a “Password reset email”.
Presumably you mean email instead of just mail, and that this is a link for resetting one's password, so I would suggest a couple of alternatives:
which will be easily understood, or
which is explicit.