As an addition to the Block List feature of the WPUM Security addon, there is now a feature that lets you allow the users to register with the email addresses only allowed in the list. You can access it in the WordPress dashboard > Users > Settings > Security tab.
This feature allows you to add addresses in full addresses(i.e. firstname.lastname@example.org), domains(i.e. @domain.com), or even partials(i.e. google). You can use a new line to separate and add multiple addresses within it.
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