msmtp is an SMTP client with a sendmail compatible interface. It can be used with Mutt and other MUAs. It forwards messages to an SMTP server which does the delivery. Features include various SMTP AUTH methods, TLS/SSL-encrypted connections (including support for client certificates), support for multiple accounts, DSN, and IPv6 support.
ADJE WebMail is a set of Perl CGI scripts which provide a Web interface to existing email accounts on any POP3-compliant email server. The software provides a complete interface, including encoding/decoding attachments, email and URL markup in received emails, and an address book for each user.
PHP Mobile Mail is aimed at mobile professionals who want to use their favorite MUAs (noteably pine or mutt) on their laptop, either in UNIX or Windows (under GNU Gygwin). This lets you work offline. It includes a mail_spooler program, which accepts the output of you MUA and spools (on, or offline). When connected, another process parses through the spool and sends your messages. It is also designed to "fake" using your REAL email address with free services like gmx.net, or mail.yahoo.com, when using their POP and SMTP servers, assuming you have a domain and account that you can control to this degree.
fdm (fetch and deliver mail) is a simple, lightweight replacement for mail fetching, filtering, and delivery programs such as fetchmail and procmail. It can fetch using POP3, POP3S, IMAP, IMAPS, or stdin, and deliver to a pipe, file, maildir, mbox, or SMTP server, based on a set of regexps. It can be used for both single user and multiuser setups, and is designed with privilege separation when running as root.
Email Security through Procmail (the Procmail Sanitizer) provides methods to sanitize email, removing obvious exploit attempts and disabling the channels through which exploits are delivered. Facilities for detecting and blocking Trojan Horse exploits and worms are also provided.