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You can limit user registration by email domains for
WordPress
WordPress Multisite
BuddyPress
WooCommerce
Paid Memberships Pro
WP Members
UsersWP
Ultimate Member
Allows you to limit new user registrations by
Domain names(e.g example.com)
Extensions (e.g. .edu, will only allow emails having .edu domain)
Email addresses (e.g admin@you-website.com, friends@some-other-website.com)
You can configure the rules to allow users in the plugin admin panel(domain/email/extensions)