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.
HashCash is an anti-spam, anti-DoS system for email. The concept centers around the sender of the mail completing a computationally expensive task (in this case, generating hash collisions) that the recipient can verify very quickly. This allows the sender to "pay" for transport in CPU cycles, creating artificial scarcity. Programs are included to add payment to messages (for the sender) and to verify message payment (for the recipient).
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).
Kerio MailServer is an email and collaboration suite that comes integrated with direct over-the-air (OTA) ActiveSync support, over 14 anti-spam layers (including SpamAssassin), dual anti-virus options (including a ClamAV plugin), collaboration features in Outlook, Entourage, Outlook Mobile, VersaMail, WebMail, and iCal, email archiving, and live automated backups. It supports a variety of SSL-encrypted communication protocols including POP3, IMAP, STMP, and HTTP for WebDAV, CalDAV, Web mail, and wireless ActiveSync. Wireless capabilities include direct email push, synchronization of calendar events, contacts, and tasks, and remote wipe for Windows Mobile devices to delete company data in the event the device is lost or stolen. Authentication options include Active Directory, Apple Open Directory, PAM, and an internal database. It can be administered remotely using an encrypted channel.
Mailonator is a system that guarantees the delivery of email attachments quickly and securely and avoiding any restrictions that might be in effect for the email recipient. Mailonator software installs on a server, so the email service and client tools never need to change. It works by intercepting email messages, stripping the attachments, and storing them on a server. In addition, Mailonator sends you an email notification when recipients download your attachment, and provides system administrators with usage statistics.
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.
Postoffice is a simple SMTP mail server and client written with spam blocking in mind. It provides a greylist to slow down the rate of incoming spam, and can be configured to do antivirus checking on incoming mail either via the sendmail milter protocol or a traditional hard-coded AV program. Postoffice can be configured to deliver mail to accounts inside vm-pop3d-style virtual domains, and it can further be enabled to support AUTH LOGIN for people inside virtual domains, so they can use postoffice as a mail forwarder when replying to mail. Postoffice accepts (and ignores) many of the same command line options that are passed to sendmail, and it comes with the usual crop of sendmail compatable aliases; runq, mailq, newaliases, and sendmail.