The Courier IMAP/POP3 maildir creation patch adds the ability to automatically create a user's home directory when he/she is successfully authenticated for the first time. It works by calling an admin-created script that takes care of creating the directory. It is quite useful in combination with authentication-modules based on network databases like MySQL or LDAP.
tMTA is a miniature Mail Transport Agent that vaguely emulates the command-line interface of Sendmail as used by many Unix programs such as mail(1). tMTA only supports local delivery to local users. It does not support SMTP or any form of inter-computer mail transport. It provides the barest functionality for daemons and users on a computer to communicate with each other using the common mail interface.
valiases mimics the sendmail behavior of evaluating aliases for mail sent to one or more virtual mail addresses. It checks whether any of the (visibly) addressed mail accounts of an email message matches one of the configured aliases, and resends mail to the alias replacement. If you don't have access to /etc/aliases but you need it, this program is for you.
The Moxy Mail Proxy acts as a mail server for multiple domains, transparently proxying mail to different hosts and/or ports. Mail servers transparently receive mail on non-standard ports (i.e., other than 25), users retain complete control over their mail server, mail bandwidth usage is logged on a per destination or per email address basis, mail can be directed to multiple servers/ports simultaneously, and more.
Hotwayd is a POP3/SMTP to HTTPMail (WebDAV) gateway. HTTPMail is a somewhat undocumented WebDAV-based protocol used by a variety of mail servers including Hotmail, MSN, Lycos, and Spray. Hotwayd allows you to use any standard POP3/SMTP mail reader to send and receive mail via these servers, similar to the way that Outlook Express can access a hotmail account. It supports features such as folder selection to get folders other than the inbox and proxy server support (if you require a proxy to access the internet).
eXtended Account Management System (XAMS) allows Administration of Unix, Web, FTP, DNS, and email accounts to manage a provider's typical userbase. Especially focusing on email account management, it enables you to easily create accounts, set aliases, and delegate domain administration to 2nd party resellers or customers. It uses PHP with MySQL or PostgreSQL as a backend, and is therefore able to manage tons of email accounts independently from your Unix/Linux accounts. Exim for SMTP and Courier for POP/IMAP are fully supported, but postfix, Cyrus, and qmail can be used too.
The Header and Envelope rewrite mini-HOWTO explains how to rewrite your from address differently in the mail header and the mail envelope. This may be necessary, for example, if your ISP requires you to send mail with one address, but you are subscribed to mailing lists with a different address, or want others to see a forwarding address instead of your ISP's address.
E-MailRelay is a simple SMTP store-and-forward mail transport agent (MTA). It runs as an SMTP server, storing incoming e-mail in a local spool directory, and then forwarding the stored messages to a downstream SMTP server on request. It can also run as a proxy server, forwarding (and optionally pre-processing) incoming e-mail as soon as it is received. It does not do any message routing other than to a local postmaster. Because of its simplicity E-MailRelay is extremely easy to configure, typically requiring only the address of the downstream SMTP server to be put on the commandline.