User Registration Plugin Blacklist.php

/* https://wordpress.org/plugins/user-registration/ */

function ur_validate_user_email( $single_form_field, $data, $filter_hook, $form_id ) {

	$value 	   = isset( $data->value ) ? $data->value : '';
	$blacklist = array( 'gmail.com', 'hotmail.com', 'outlook.com', 'aol.com', 'mail.com', 'yahoo.com', 'icloud.com' );
	$parts     = explode( '@', $value );

	if( in_array( $parts[1], $blacklist ) ) {

	    add_filter( $filter_hook, function ( $msg ) use ( $field_label ) {
	        return __( 'You must use a business email address. Email addresses like @gmail.com and @outlook.com are not accepted.', 'user-registration' );
	   });
	}
}

add_action( 'user_registration_validate_user_email','ur_validate_user_email', 10, 4 );