Blocking outside Roundcube emails

[Sheba]

How do I block emails from being received in Roundcube from sender not within the intranet setup?  We have a membership site and want only members to be able to send emails to each other and not spammed.

For example if the domain is Happy Harry's Pharmacy | Walgreens and 2 emails ([email protected] and [email protected]) exist.  If my email is [email protected] and I send emails to either or both happy.com emails, it should be blocked, not received by either email.

Thanks for your attention.

Sheba

[SKaero]

It would have to be done by your mail server.