block certain email domains from registering

Hi,

This will only be possible with some custom PHP coding. I do not advice to go this route if you are not an experienced PHP developer.

I would suggest your own custom validation, here is an example:
https://github.com/ibericode/mc4wp-snippets/blob/master/forms/custom-validation.php

If you use Premium version of our plugin we will be able to provide a little more support on this.

There also is a hook for the form data if you like to go that route:
http://developer.mc4wp.com/reference/hooks/mc4wp_form_data/