SpamFilter will not know if a mailbox exists or not until it has finished receiving the email and tries then to forward it to your SMTP server. There is an "Authorized TO" whitelist in SpamFilter however. This allows you to add to SpamFilter the full list of email addresses valid on your system. SpamFilter will only accept emails if the recipients are on that list, and will reject any email whose recipient is not on the list.

Ric,

We always steered away from SpamFilter having to perform database lookups for its filters. The two main reasons are (in order): (1) performance - database queries are *much* slower than how we're currently handling them with text files. (2) reliability - we do not want to risk loosing mail when the database server goes down. For sites that process millions of emails daily (1) is of great concern, so we've set that option aside for now.