MessageWall is a free software SMTP proxy. It sits between the outside world and your mail server and keeps out viruses, spam and mail relaying. Unlike many existing ISP-level filtering solutions, it offers customization of filtering features on a per-address basis. MessageWall employs a scoring system that allows message rejection based on multiple rules with different weights, and header tagging when the message doesn't reach the threshold.

MessageWall also supports strong relay authentication based on source address or SMTP AUTH PLAIN or LOGIN methods, as well as SSL/TLS connections from clients and to the backend server.

MessageWall organizes filtering customizations into profiles. The configuration file defines a default profile, and a seperate file specifies addresses and domains with profiles other than the default. Each profile contains a set of rules for how to filter mail. The available rules are:

• Virus scanning based on Open Anti-Virus pattern files
• DNS-based blacklist IP checking
• DNS-based blacklist source address domain checking
• DNS-based distributed checksum clearinghouse checking
• Source address domain MX/A record checking
• IP reverse DNS checking
• To: header and destination address matching
• From: header and source address matching
• From: header real name checking
• Arbitrary header rejection (case sensitive or insensitive)
• Arbitrary body text rejection (after MIME decoding, case sensitive or insensitive)
• MIME part filename matching
• MIME part rejection by type
• MIME part stripping by type
• SMTP error counting and maximum