Restrict membership signup by email domain only for set membership levels.

my-pmpro-restrict-checkout-email-domain-per-level.php

<?php
/**
 * Restrict Membership Signup by Email Domain for set levels.
 * Make sure to edit the $restricted_levels and $valid_domains array defined
 * further below to include only the domains you'd like to allow.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function my_pmpro_registration_checks_restrict_email_addresses_per_level( $continue_registration ) {

	// Set your restricted levels here.
	$restricted_levels = array( 1, 2, 3 ); // IDs of levels to restrict

	$level    = pmpro_getLevelAtCheckout(); // Get the level object.
	$level_id = intval( $level->id ); // Get the level ID.

	if ( ! in_array( $level_id, $restricted_levels ) ) {
		return $continue_registration; // Bail if the level is not restricted.
	}

	// Do we have an email?
	if ( ! isset( $_REQUEST['bemail'] ) || empty( $_REQUEST['bemail'] ) ) {
		return $continue_registration; // Bail if there is no email.
	}

	$email = wp_unslash( $_REQUEST['bemail'] ); // Get the email.

	if ( ! my_checkForValidDomain( $email ) ) {
		pmpro_setMessage( 'Please enter a valid email address', 'pmpro_error' );
		return false;
	}

	return $continue_registration;
}
add_filter( 'pmpro_registration_checks', 'my_pmpro_registration_checks_restrict_email_addresses_per_level', 10, 1 );

//Taken from: http://www.bitrepository.com/how-to-extract-domain-name-from-an-e-mail-address-string.html
function my_getDomainFromEmail( $email ) {
	// Get the data after the @ sign
	$domain = substr( strrchr( $email, '@' ), 1 );

	return $domain;
}

function my_checkForValidDomain( $email ) {
	$domain = my_getDomainFromEmail( $email );

	// Update this array to include the domains you want to allow.
	$valid_domains = array( 'yahoo.com', '*.gmail.com', '*.domain.uk' );

	foreach ( $valid_domains as $valid_domain ) {
		$components      = explode( '.', $valid_domain );
		$domain_to_check = explode( '.', $domain );

		if ( $components[0] == '*' && count( $domain_to_check ) > 2 ) {
			if ( $components[1] == $domain_to_check[1] && $components[2] == $domain_to_check[2] ) {
				return true;
			}
		} else {
			if ( ! ( strpos( $valid_domain, $domain ) === false ) ) {
				return true;
			}
		}
	}

	return false;
}