Tagged Message Delivery Agent
TMDA is an open-source software application designed to reduce the amount of spam (Internet junk-mail) a user receives. TMDA's main difference from other anti-spam systems is a challenge/response system that bulk mailing machines and programs are either unwilling (to save bandwidth) or unable (due to lack of programming) to answer.The technical countermeasures used by TMDA to thwart spam include:
- Whitelists: Whitelists are essentially an 'accept list'. All messages from addresses in these lists are automatically allowed through no matter what the content is.
- Blacklists: Blacklists are a 'block list'. All messages from addresses in these lists are automatically blocked.
- Challenge/Response: Allows senders that are not on either the Whitelist or the Blacklist to prove that they are legitimate (not spam). This can be an image with a response question or some other script that a mass-mailing machine is unwilling or unable to do.
- Tagged addresses: Special-purpose e-mail addresses such as time-dependent addresses, or addresses which only accept certain kinds of communication. These increase the transparency of TMDA for unknown senders by allowing them to safely circumvent the challenge/response system.