The student form validation rule for email length is still set to 75 characters (https://github.com/edx/edx-platform/blob/master/common/djangoapps/student/forms.py#L132-L139), this used to be because of a restriction on Django 1.4.
But since Dogwood uses Django 1.8 this rule can now be set to 254 characters (https://docs.djangoproject.com/en/1.8/ref/models/fields/#emailfield), as auth_user.email field is now a varchar(254).
1. Access the Sign Up page
2. Enter an email address longer than 75 characters, such as "email@example.com". Fill the other required field.
3. You will receive this error message: "Email cannot be more than 75 characters long"