others are:
SMTP
AfterLogic =WebMail Lite Open Source SMTP server for windows.
agorum core= Open Source Document Management/Enterprise Content Management
Atmail
Axigen
Bongo
Citadel
ContactOffice
CommuniGate Pro
Courier
Eudora Internet Mail Server
Exim
FirstClass
fmaild
Gammadyne Mailer
Gordano Messaging Suite
HMailServer
IBM Lotus Domino
Ipswitch IMail Server
James (Java Apache Mail Enterprise Server)
Kerio Connect
MagicMail
Mailtraq
MDaemon Email Server for Windows by Alt-N Technologies
Mercury Mail Transport System
Meldware Communication Suite
MeTA1 (successor of the sendmail X project)
Microsoft Exchange Server
MMDF
Novell GroupWise
Novell NetMail
Openwave Systems
Open-Xchange
Oracle Beehive
Oracle Communications Messaging Exchange Server
Postfix
qmail
qpsmtpd
Scalix
Sendmail
Smail
SparkEngine
SMTP Proxy
Sun Java System
Synovel Collabsuite ( Linux mail server )
WinGate
XMail
XMS Email Application Server
[[Zarafa (software)|Zarafa]]
Zimbra
ZMailer
POP/IMAP
agorum core Open Source Document Management/Enterprise Content Management
Alt-N Technologies' MDaemon Email Server for Windows
Apache James
Axigen
Bongo
Citadel - uses a database-driven mail store
CommuniGate Pro
ContactOffice
Courier Mail Server - uses Maildir format
Cyrus IMAP server
DBMail
Dovecot - uses Maildir, legacy mbox, and experimental dbox format
Eudora Internet Mail Server
FirstClass
Gordano Messaging Suite
HMailServer
Ipswitch IMail Server
Kerio Connect
IBM Lotus Domino IMAP4 Server
MagicMail
Mailtraq
Meldware Communication Server (Free open source multi-platform mail server)
Mercury Mail Transport System
Microsoft Exchange Server
Microsoft Windows POP3 Service
Novell GroupWise
Novell NetMail
Open-Xchange
Oracle Beehive
Synovel Collabsuite (IMAP & POP with Calendar, Chat, anti-spam, Archival, data backup)
UW IMAP - uses mix, mbx, and legacy mbox format.
WinGate
XMail
Zarafa
Zimbra
Technologies' MDaemon Email Server for Windows
ASSP
Axigen
Bayesian filters
Bogofilter
DSPAM
Gordano Messaging Suite
Hexamail Guard
Kerio Connect
MagicMail
MagicSpam
MailChannels
MailScanner
Mailtraq
MIMEDefang
Procmail
PureMessage
SurfControl
SpamAssassin
WinGate
Vipul's Razor
Webroot