mypmpro-restrict-user-emails-partially.php

<?php //do not copy

/**
 * This recipe allows you to restrict partial user email addresses during checkout.
 * You will be able to specify that all @paidmembershipspro.com email addresses are allowed to sign up, without
 * having to enter each user's full email address. 
 * 
 * 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 mypmpro_remove_actions() {

	remove_filter( "pmpro_registration_checks", "pmprorh_pmpro_registration_checks" );

}
add_action( 'init', 'mypmpro_remove_actions' );

/**
 * Registration checks the restricted email domain
 */
function mypmprorh_pmpro_registration_checks( $okay ) {

	global $current_user;

	//only check if we're okay so far and there is an email to check
	if( $okay && ( !empty($_REQUEST['bemail'] ) || !empty( $current_user->user_email ) ) ) {
		//are we restricting emails for this level
		global $pmpro_level;
		
		$restrict_emails = pmpro_getOption("level_" . $pmpro_level->id . "_restrict_emails");
		
		if( ! empty( $restrict_emails ) ) {
			
			$restrict_emails = strtolower( str_replace( array( ";", ",", " " ), "\n", $restrict_emails ) );
			
			if( ! empty( $current_user->user_email ) ) {
				$needle = strtolower( $current_user->user_email );
			} else {
				$needle = strtolower( sanitize_email( $_REQUEST['bemail'] ) );
			}
			
			$haystack = explode( "\n", $restrict_emails );

			array_walk( $haystack, function( &$val ) { 
				$val = trim($val);
				return $val;
			});

			$okay = false;
			foreach( $haystack as $hay ) {
				
				if ( strpos( $needle, $hay ) !== false ) {
					//Needle found in haystack - do nothing
					$okay = true;
				}
			}

			if( ! $okay ) {
				global $pmpro_msg, $pmpro_msgt;

				$pmpro_msg = "This membership level is restricted to certain users only. Make sure you've entered your email address correctly.";
				
				$pmpro_msgt = "pmpro_error";

				return $okay;
			}

		}

		//are we restricting user names for this level
		$restrict_usernames = pmpro_getOption("level_" . $pmpro_level->id . "_restrict_usernames");

		if( ! empty( $restrict_usernames ) ) {

			$restrict_usernames = strtolower( str_replace( array( ";", ",", " " ), "\n", $restrict_usernames ) );

			if( ! empty( $current_user->user_login ) ) {
				$needle = strtolower( $current_user->user_login );
			} else {
				$needle = strtolower( $_REQUEST['username'] );
			}

			$haystack = explode( "\n", $restrict_usernames );

			array_walk( $haystack, function( &$val ) { 
				$val = trim($val);
				return $val;
			});

			if( ! in_array( $needle, $haystack ) ) {

				global $pmpro_msg, $pmpro_msgt;

				$pmpro_msg = "This membership level is restricted to certain users only. Make sure you are logged into your existing account and using the proper username.";
				
				$pmpro_msgt = "pmpro_error";
				
				$okay = false;
			}
		}
	}

	return $okay;
}
add_filter( "pmpro_registration_checks", "mypmprorh_pmpro_registration_checks" );

Need to update the foreach on line 42. I've forked it here with the change https://gist.github.com/kimwhite/dcbee33061df3429fff529ad1304e57a
(thanks to DavidP for the help!)