Heirloom mailx (formerly known as "nail") is derived from Berkeley Mail and provides the functionality of the System V and POSIX mailx commands. Additional features include support for MIME, IMAP (including caching and disconnected use), POP3, SMTP, S/MIME, international character sets, maildir folders, message threading, powerful search methods, scoring, and a Bayesian junk mail filter. Mailx can be used as a mail batch language in nearly the same way as it is used interactively. It can thus act as a mailbox filter, can fetch mail from remote accounts, and can send files as attachments.
NeoMail is a web-based email client written in Perl. It provides access to a machine's local mail spools without requiring a POP3 server to be running, and without giving users shell access. It supports sending and receiving attachments, multiple folders, user themes and preferences, templates for interface generation, quotas, language translations, etc.
Netbiff is an advanced version of the traditional biff (a mail-checking program). Features include BSD mailbox and IMAP4 support, a backend plug-in system (for other protocols), arbitrary numbers of backends, pluggable interfaces (GTK and text-based), and arbitrary actions on biff.
nmh (new MH) is a powerful electronic mail handling system. It was originally based on version 6.8.3 of the MH message system developed by the RAND Corporation and the University of California. It is intended to be a (mostly) compatible drop-in replacement for MH. nmh consists of a collection of fairly simple single-purpose programs to send, receive, save, retrieve, and manipulate e-mail messages. Since nmh is a suite rather than a single monolithic program, you may freely intersperse nmh commands with other commands at your shell prompt, or write custom scripts which use these commands in flexible ways.
NOCC is a simple and fast Web-based e-mail reader which can handle POP3, SMTP, and IMAP servers. It is written with PHP5, and uses sessions, has low requirements (no database, frames), and is easy to install. It also features multi-language support, MIME attachment decoding/encoding, and the ability to display HTML messages.
NS WebMail offers standard mail functions for POP3 inbox management. Mail is sent using SMTP protocol. It has been designed to be light and simple, to avoid having to use heavy IMAP or SQL servers, and it has full MIME support for incoming and outgoing mail. It is Perl-strict and mod_perl-compliant, and has security using HTTP authentication or cookies. LDAP connectors are also available (as optional plug-ins) for authentication and read-only address book. Two interfaces are available (with and without frames). It is able to recognize tags from spam detection software (Spamassassin is supported out of the box).