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The Courier mail transfer agent (MTA) is an integrated mail/groupware server based on open commodity protocols such as ESMTP, IMAP, POP3, LDAP, SSL, and HTTP. Courier provides ESMTP, IMAP, POP3, Webmail, calendaring, and mailing list services.
E-MailRelay is a simple SMTP proxy and relay MTA, written in C++, for Linux and Windows. As a proxy all messages can be passed through a user-defined program, such as a spam-filter, which can drop, readdress, or edit messages as they pass through.
jumfi will provide a java based uniform mail filter interface using C, JNI and Java. It will allow mail filters build in java to be applied to multiple mail servers of similar and different design.
Spey is the easy way to reduce the amount of spam you receive. It sits between your mail server and the outside world and performs greylisting and other verification checks. It needs very little configuration and no mail server setup and Just Works(TM).
AppSignal's MCP server hands Claude, Cursor, or Zed your real errors, traces, and the deploy that shipped them. AI writes the fix; you review the diff.
smtptrapd is a multi-threaded daemon written in C that provides a RFC 2821 compliant SMTP service that always returns a 4xx soft error to the RCPT TO verb. It can be deployed as a secondary MX to reduce the amount of inbound spam.
Tools for a mail delivery service, including LMTP and POP3 servers and also getpwnam() emulation. Mail is stored in Maildir++ format (qmail plus courier extensions as soft quota and subfolders). User defined filter and virtual domains are supported.
tMTA - A tiny|trivial MTA tMTA (pronounced 'tomata', like 'tomato' :) is a miniature Mail Transport Agent. tMTA only supports local delivery to local users. It does not support SMTP or any form of inter-computer mail transport.
This is a POP3 to SMTP bridge that will use the SMTP Trace headers to determine who it's from and to. Designed to work with catch all POP3 accounts. Configuration for the POP3 and SMTP accounts are in a CSV file.
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FMTA is a simple SMTP/POP3 server designed to run on Fox boards (Fox board is a small but powerful embedded computer running a complete GNU/Linux system).
This project aims to implement in-line filtering functionality for Sendmail. This will allow filters to process mail data on the wire (i.e. earlier). This is primarily intended for better status reports of in-transit mails.
elspy is an extension to the Exim MTA that allows Exim users to supply a local_scan() function written in Python instead of C. (Exim's local_scan() API lets sysadmins supply site-specific code for approving/rejecting email messages at SMTP time.)
dkfd is a DomainKeys filter daemon using libdomainkeys library. targeted usage is SMTP proxy of Postfix MTA. Currently developing under Linux/FreeBSD, and Solaris.
Virtual Qmail - virtual domains management system for qmail. It is designed to be fast, reliable, and easy to use. Includes software allowing users to manage their accounts via WWW.
Syron Qmail is a qmail distribution based on netqmail and qmail-sql
which tries to unify several public available patches to include almost
all modern mail server features into qmail (SQL User Database, SMTP
AUTH/TLS, high load adjustments and more).