bbPress: whitelist email domains for anonymous topic subscriptions?

I manage a bbPress-based student tech help forum for a college; site registrations are disabled (and must remain disabled). I have anonymous subscriptions/email notifications enabled, and email addresses are required for the creation of replies or topics, but am looking for a way to block any but a few email address domains from creating topics/replies. Is there a way to do this without enabling registrations? The only whitelisting options I’ve been able to find apply to registration, not anonymous subscriptions/topic and reply creation.