For mail architectures that are highly distributed, consolidating mails and running elgibility-to-send to certain addresses is difficult and problematic.

Mailguard is designed such that addresses can be routed to a central server. Upon reaching the server, the data is sent via STDOUT into the mailguard application.

The application then classifies the sender and based on configuration parameters decides whether the mail should be silently discarded, forwarded on, or put into a holding pen until the message can be validated by a moderator.

This solution was originally written at Cisco Inc. by Steven Harms (http://www.stevengharms.com) but was granted release into the public domain under an open-source license.