Absolutely yes! You can use the "Authorized TO" email whitelist.

If you have an "Authorized TO" list, SpamFilter will only accept an email address if it is in that list. Any attempt to send an email to a recipient not on that list will cause a 550 error, followed by an immediate forced disconnect of the sender (preventing them from attempting dictionary attacks by trying multiple addresses...).

The file containing the "Authorized TO" entries is continuosly monitored for changes (as are *all* configuration files in SpamFilter). If the file is modified outside of SpamFilter, it will be automaticllay re-imported within 60 seconds. This allows you to simply export a list of your users with any schedule you like. SpamFilter will automatically reload it every time.

As a side note, every time SpamFilter rejects an email because it is spam, it will always do so by sending a 5xx error to the sender, which, as you probably guessed already, means that SpamFilter will never send an NDR itself, thus saving bandwidth and resources. The only time SpamFilter sends an NDR is if accepts a "good" email, but then encounters problems when trying to forward it to your SMTP server (as in "mailbox full", "non existent user" etc.). In this case SpamFilter does have to notify the sender, and will have to send them an NDR.