getmail is intended as a simple, secure, and reliable replacement for fetchmail. It retrieves email (either all messages, or only unread messages) from one or more POP3, SPDS, or IMAP4 servers (with or without SSL) for one or more email accounts, and reliably delivers into qmail-style Maildirs, mboxrd files, or through external MDAs (command deliveries) specified on a per-account basis. getmail also has excellent support for domain (multidrop) mailboxes, including delivering messages to different users or destinations based on the envelope recipient address.
Smail is a Mail Transport Agent. Its job is to accept mail messages from sources on the local machine, or from remote hosts, and deliver those messages to the appropriate destinations, be they to remote hosts or to files or programs on the local machine. Smail is designed to be mostly compatible with sendmail but uses a much simpler configuration scheme.
The deepOfix Mail Server makes it possible for organizations with minimal technical expertise to have robust server infrastructure. deepOfix offers unprecented ease of use, doesn't require prior GNU/Linux experience and is backed by a full-time development and support team. deepOfix features LDAP-based directory services for user account management, email delivery, distribution lists, and addressbooks. It also includes built in tools for spam and virus control, webmail, and fetching email.
IVS Milter is a virus, spam, and content milter. It makes use of the antivirus package ClamAV, and can also seamlessly integrate SpamAssassin (either spamc or libspamc). IVS Milter is also plugin based and currently supports virus scanning, spam scanning, content-type matching, attachment type matching, and attachment filename matching.
BitDefender Mail Protection is a powerful mail filtering solution that provides antivirus and antispam capabilities to Linux mail servers. Both sent and received messages are scanned in real-time, avoiding possible infections and preventing anyone from sending an infected message. It claims a 100% detection rate for all viruses in the wild through its powerful scanning engines, which are certified by prestigious testing labs.
spfmilter is a milter (sendmail mail filter) which implements the Sender Policy Framework (SPF). SPF is a strategy for preventing junk mail. The SMTP standard for email allows anyone to forge anyone else's email address. SPF verifies that the Sender address of an email message matches (according to some policy) the client IP address that submitted it.
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.