Block restricted specific email addresses from checking out for a membership.

block-restricted-email-addresses-pmpro-registration-checks.php

<?php
/**
* Exclude specific email domains from membership signup.
* Make sure to edit the $invalid_domains array defined further below
* to include only the domains you'd like to block.
*
*/
function block_restricted_email_addresses_pmpro_registration_checks( $value ) {

	// set array of restricted email addresses.
	$restricted_emails = array( '[email protected]', '[email protected]' );

	// do we have an email to check?
	if ( ! isset( $_REQUEST['bemail'] ) || empty( $_REQUEST['bemail'] ) ) {
		return $value;
	}

	// get email address
	$email = $_REQUEST['bemail'];

	if ( in_array( $email, $restricted_emails ) ) {
		global $pmpro_msg, $pmpro_msgt;
		$pmpro_msg  = 'Please enter a valid email address';
		$pmpro_msgt = 'pmpro_error';
		return false;
	}

	return $value;
}
add_filter( 'pmpro_registration_checks', 'block_restricted_email_addresses_pmpro_registration_checks', 10, 1 );

To exclude an email domain from checking out see https://www.paidmembershipspro.com/exclude-certain-email-domains-from-membership-signup/