Sendmail is a mail transfer agent (MTA) originally developed by Eric Allman. It is highly configurable and supports many delivery methods and many transfer protocols.

OfflineImap synchronizes emails between two repositories, so that you can read the same mailbox from multiple computers. It supports IMAP as REMOTE repository and Maildir/IMAP as LOCAL repository.

The mblaze message system is a set of Unix utilities for processing and interacting with mail messages which are stored in maildir folders.

Its design is roughly inspired by MH, the RAND Message Handling System, but it is a complete implementation from scratch.

mblaze is a classic command line MUA and has no features for receiving or transferring messages; you can operate on messages in a local maildir spool, or fetch your messages using fdm(1), getmail(1), offlineimap(1), or similar utilities, and send it using dma(8), msmtp(1), sendmail(8), as provided by OpenSMTPD, Postfix, or similar.

mblaze operates directly on maildir folders and doesn't use its own caches or databases. There is no setup needed for many uses. All utilities have been written with performance in mind. Enumeration of all messages in a maildir is avoided unless necessary, and then optimized to limit syscalls. Parsing message metadata is optimized to limit I/O requests. Initial operations on a large maildir may feel slow, but as soon as they are in the file system cache, everything is blazingly fast. The utilities are written to be memory efficient (i.e. not wasteful), but whole messages are assumed to fit into RAM easily (one at a time).

Mairix is a program for indexing and searching email messages stored in Maildir, MH, MMDF or mbox folders.

This project is a reimplementation of the Python stdlib smtpd.py based on asyncio.

alterMIME is a small program which is used to alter your mime-encoded mailpack. What can alterMIME do?

1. Insert disclaimers,

2. insert arbitrary X-headers,

3. modify existing headers,

4. remove attachments based on filename or content-type,

5. replace attachments based on filename.

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Mail::AuthenticationResults parses the message header field that indicates the message authentication status as per RFC7601. This module is not fully compliant with the RFC but it tries to implement most styles of Authentication-Results header seen in the wild.

isync/mbsync is a command-line tool for two-way synchronization of mailboxes. Currently Maildir and IMAP are supported types.

Mlmmj is a simple and slim mailing list manager (MLM) inspired by ezmlm. It works with many different Mail Transport Agents (MTAs) and is simple for a system administrator to install, configure and integrate with other software. As it uses very few resources, and requires no daemons, it is ideal for installation on systems where resources are limited. Its features include:

1. Archive, Custom headers / footer,

2. Fully automated bounce handling (similar to ezmlm),

3. Complete requeueing functionality, Moderation functionality, Subject prefix,

4. Subscribers only posting, Regular expression access control,

5. Functionality to retrieve old posts, Web interface, Digests,

6. No-mail subscription, VERP support,

7. Delivery Status Notification (RFC1891) support,

8. Rich and customisable texts for automated operations.

NeoMutt is a command-line mail reader which is based on Mutt. It adds a large amount of new and improved features to Mutt.

Technology for Resting Email Encrypted Storage (TREES) is a NaCL-based Dovecot encryption plugin. This plugin adds individually encrypted mail storage to the Dovecot IMAP server. It is inspired by Posteo's scrambler which uses OpenSSL and RSA key pairs. TREES works in a similar way, but uses the Sodium crypto library (based on NaCL).

How it works:

1. On IMAP log in, the user's cleartext password is passed to the plugin.

2. The plugin creates an argon2 digest from the password.

3. This password digest is used as a symmetric secret to decrypt a libsodium secretbox.

4. Inside the secretbox is stored a Curve25519 private key.

5. The Curve25519 private key is used to decrypt each individual message, using libsodium sealed boxes.

6. New mail is encrypted as it arrives using the Curve25519 public key.

A schema-driven command line tool for administering Stalwart Mail and Collaboration Server over its JMAP API.

Alot is a terminal-based mail user agent based on the Notmuch mail indexer. It is written in Python using the urwid toolkit and features a modular and command prompt driven interface to provide a full mail user agent (MUA) experience as an alternative to the Emacs mode shipped with Notmuch.

Mailman3 allows emails sent to its mailing lists to be archived by any software provided that there is a plugin (loadable by Mailman3) designed to communicate with it properly. This module contains a Mailman3 archiver plugin which sends emails to HyperKitty, the official Mailman3 web archiver.

The RSS2email program (r2e) fetches RSS/Atom news feeds, converts them into emails, and sends them.

Sieve-connect lets you view, upload, edit, delete, and otherwise manage Sieve scripts on any mail server that speaks the ManageSieve protocol, as specified in RFC 5804.

Sieve (RFC 5228) is a specialised language for e-mail filtering. Sieve scripts are stored on the server and run whenever mail arrives. They can automatically sort new messages into folders, silently reject them, send an automated response, and more.

sieve-connect is designed to be both a tool which can be invoked from scripts as well as a decent interactive client. It supports TLS for connection privacy, as well as authentication with SASL or GSSAPI client certificates. It should be a drop-in replacement for sieveshell from the Cyrus IMAP project.

This Python module can be used to generate and parse RFC 5451/7001/7601 Authentication-Results email headers. It supports extensions such as:

• RFC 5617 DKIM/ADSP

• RFC 6008 DKIM signature identification (header.b)

• RFC 6212 VBR

• RFC 6577 SPF

• RFC 7281 Authentication-Results registration for S/MIME

• RFC 7293 The Require-Recipient-Valid-Since header field

• RFC 7489 DMARC

• ARC (draft-ietf-dmarc-arc-protocol-08)

Balsa is a highly configurable and robust mail client for the GNOME desktop. It supports both POP3 and IMAP servers as well as the mbox, maildir and mh local mailbox formats. Balsa also supports SMTP and/or the use of a local MTA such as Sendmail.

A third-party, open-source ProtonMail bridge. It is designed to run on a server. It supports CardDAV, IMAP and SMTP.

The sendgmail command provides a minimal sendmail-compatible front-end that connects to Gmail using OAuth2. It is specifically designed for use with git send-email. The command needs a Gmail API key to function.

Guix's version of sendgmail has been patched for compatibility with all known forks, including support for non-@gmail.com email addresses.

public-inbox implements the sharing of an email inbox via Git to complement or replace traditional mailing lists. Readers may read via NNTP, IMAP, Atom feeds or HTML archives.

CRM114 is a system to examine incoming e-mail, system log streams, data files or other data streams, and to sort, filter, or alter the incoming files or data streams according to the user's wildest desires. Criteria for categorization of data can be via a host of methods, including regexes, approximate regexes, a Hidden Markov Model, Orthogonal Sparse Bigrams, WINNOW, Correlation, KNN/Hyperspace, or Bit Entropy (or by other means--it's all programmable).

nmail is an easily configurable terminal-based email client with a ncurses user interface similar to alpine and pine.

Mu is a tool for dealing with e-mail messages stored in the Maildir format. Mu's purpose in life is to help you to quickly find the messages you need; in addition, it allows you to view messages, extract attachments, create new maildirs, and so on.