sendqmailq is intended for home computers that run qmail as their MTA but do not have permanent connection to the Internet. The outbound emails are stored locally in a queue, and when the IP connection is brought up, sendqmailq sends them to the ISP's SMTP server. Unlike serialmail, sendqmailq sends one single copy of an email to multiple recipients (instead of one copy each).
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.
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.
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.
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).
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.
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.