AqMail fetches mails from remote mailboxes (POP3), applies admin-defined filters, and stores the email in local mailboxes for POP3 daemons to serve them. It supports virtual mail domains and users. It has been tested with QMail as the MTA and with Spamassassin and ClamAV as filters.
PEAR Validate is a set of useful methods to validate various kinds of data. It supports numbers (min/max, decimal, or not), email addresses (syntax, domain check, RFC 822), strings (alpha, upper and/or lowercase, numeric), dates (min, max), URIs (RFC 2396), and more. It also has many locale validation rules, specialized for each country or region (US, FR, UK, DE) or application domain (such as finance).
libspawner implements the MTA side of Sendmail's Milter (mail filter) protocol that is used by the MTA to delegate policy decisions to external daemons. The main purpose of this library is to enable other MTAs, or any mail-handling software, to talk to the existing mail filters that have been developed for Sendmail.
Sporkie is a challenge/response spam email filter. It uses CAPTCHAs (randomly generated, distorted images of numbers) in order to verify that a person sending email is human. It works in the same manner as the ASK project, except that Sporkie uses CAPTCHAs. The main advantage to using CAPTCHAs is that it catches automated spam bots that don't spoof their email address. With ASK, such bots would get through the system. Sporkie is provided under the OSCL Type-C license.