Jsmtpd is a small and modular SMTP server daemon. It is heavily based on plugins, and currently supports spam detection (Spamassassin, Jasen, RBL servers, and BlackList), anti-virus (Clam Antivirus), Recipient rewriting, and many other filters. Filtering is done with a customisable tree of filters. Almost all functionality is implemented as plugins, making it easy to fit your needs.
The QmailToaster is an RPM-based Qmail distribution for CentOS, Fedora, SuSE, and Mandriva. This system is designed for ease of deployment and includes strong anti-spam systems. This includes support for SRS, SPF, DomainKeys, and virtual hosting on the SMTP side. On the user side, this has a Web-based administration interface and supports POP3, POP3 over SSL, IMAP, and IMAP over SSL.
perl-esmtpd is an antispam, antivirus, and authenticating SMTP daemon. It integrates with qmail, sendmail, stunnel, etc. It's simple Perl, so you can hack it to suit your needs. It supports RBLs, mx on from, bounce evil phrases, virus scanning, etc. It offers: RBL-sytle black-hole DNS checking when not relaying, verification of valid MX records for the MAIL FROM (when not relayin, the ability to use Stunnel for smtps connections and still do RBL checks etc., simple authentication (i.e. PLAIN or LOGIN) for offsite users, and the ability to reject email with certain key phrases. Many other useful features are included.
JeguePanel is a panel to provide easy management of GNU/Linux servers. It makes services configuration (email and samba) much more easy and efficient, and handles user and permissions management. It's integrated with monitoring tools and a lot of control programs like anti-virus, anti-spam, reports, etc. It has English, Portuguese, and Spanish translations. It can use OpenLDAP, PostgreSQL, or MySQL as a backend, and has full integration with Active Directory. It will show different interfaces to users based on permission levels.
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.
mysqmail is a replacement for the qmail standard checkpasswd that performs authentication with a MySQL table. After authentication, it sets up two environment variables that it uses to perform traffic accounting. For SMTP, it provides an alternative logger for the qmail-send program. Instead of logs, it writes the traffic to a MySQL table for the related domain.
UMPL for XmailServer is a Web frontend for managing Xmailserver and Webmailsolution. Different frontends are presented for root, the postmasters of virtual domains, and end-users. It includes support for managing finger, POP3, SMTP, and Webmail in German, English, Dutch, Italian, and Spanish.