Comparison of mail servers explained

The comparison of mail servers covers mail transfer agents (MTAs), mail delivery agents, and other computer software that provide e-mail services.

Unix-based mail servers are built using a number of components because a Unix-style environment is, by default, a toolbox[1] operating system. A stock Unix-like server already has internal mail; more traditional ones also come with a full MTA already part of the standard installation. To allow the server to send external emails, an MTA such as Sendmail, Postfix, or Exim is required. Mail is read either through direct access (shell login) or mailbox protocols like POP and IMAP. Unix-based MTA software largely acts to enhance or replace the respective system's native MTA.

Microsoft Windows servers do not natively implement e-mail, thus Windows-based MTAs have to supply all the necessary aspects of e-mail-related functionality.

Feature comparison

License
Mail serverLinux/UnixWindowsmacOSSMTPPOP3IMAPJMAPIDLESMTPSPOP3SIPv6SSLDANEWebmailActiveSyncSieveIDN/UTF8
agorum core
Apache James
Axigen (free version: 5 domains, 5 users, 5 groups)
Citadel
CommuniGate Pro (Free for up to 5 users)
Courier Mail Server[2]
Cyrus IMAP[3] [4]
DBMail
Dovecot[5] [6] [7]
Eudora Internet Mail Server
Exim (via Cygwin)[8] [9]
FirstClass
Gordano Messaging Suite
GroupWise
Halon[10]
Haraka
hMailServer
HCL Domino [11]
IceWarp Mail Server
Ipswitch IMail Server (since v12.2)[12]
Kerio Connect
Kolab
Kopano[13]
KumoMTA
Mailsite
Mailtraq
MDaemon
Mercury Mail Transport System
Microsoft Exchange Server[14] [15] [16] 3rd Party support
NetMail
OpenSMTPD[17]
Open-Xchange[18]
Oracle Communications Messaging Server
Postfix
qmailwith with patchwith , via Qsmtpwith with patch
Qpopper (via Cygwin)[19]
Scalix (via Stunnel) (via Stunnel) (via Stunnel)
Sendmail (since 8.16)[20]
SparkEngine
UW IMAPWebPine (Indexes)[21]
WinGate
Zarafa
Zentyal
Zimbra[22]
Mail serverLinux/UnixWindowsmacOSSMTPPOP3IMAPJMAPIDLESMTPSPOP3SIPv6SSLDANEWebmailActiveSyncSieveIDN/UTF8DatabaseFile-basedLicense

Authentication

Mail serverSMTP AUTHPOP before SMTPAPOPFile SystemDatabaseLDAPNative Anti Hammer - Brute-force attack protectionOther
agorum corePLAIN, LOGIN, CRAM-MD5,[23] NTLM
Apache JamesPLAIN, LOGIN
AxigenGSSAPI, CRAM-MD5, DIGEST-MD5,[24] APOP, CLRTXT, LOGIN, PLAIN, LDAP, Internal
CommuniGate ProActive Directory (LDAP), CLRTXT, CRAM-MD5, DIGEST-MD5, APOP, GSSAPI, NTLM, MSN, SESSIONID, External Authentication Plug-in. Also supports X.509 PKI auth via STARTTLS/SSL
Courier Mail ServerManaged by Courier authentication library which can use PAM, Userdb, PostgreSQL(beta), MySQL, LDAP, and EXTERNAL.
Cyrus IMAPSupports X.509 PKI auth via STARTTLS and EXTERNAL. Uses Cyrus SASL authentication library for support for other mechanisms: Berkeley DB, GDBM, or NDBM (sasldb), PAM, MySQL, PostgreSQL, SQLite, LDAP, Active Directory(LDAP), DCE, Kerberos 4 and 5, proxied IMAP auth, getpwent, shadow, SIA, Courier Authdaemon, httpform, APOP and SASL mechanisms: ANONYMOUS, CRAM-MD5, DIGEST-MD5, SCRAM-SHA-1(-PLUS), SCRAM-SHA-224(-PLUS), SCRAM-SHA-256(-PLUS), SCRAM-SHA-384(-PLUS), SCRAM-SHA-512(-PLUS), EXTERNAL, GSSAPI, LOGIN, NTLM, OTP, PASSDSS, PLAIN, SRP
DBMailLOGIN, Active Directory, OpenLDAP
DovecotSubmission service supports SMTP AUTH. Offers SASL to MTAs.[25] [26] POP-before-SMTP via DRAC plugin. PAM, MySQL, PostgreSQL, SQLite, LDAP, Active Directory(LDAP), Kerberos 5, proxied IMAP auth, getpwent, shadow, SIA, BSDauth, Vpopmail. APOP and SASL mechanisms: ANONYMOUS, PLAIN, LOGIN, CRAM-MD5, DIGEST-MD5, SCRAM-SHA-1, EXTERNAL, GSSAPI, NTLM, OTP, SKEY, RPA.
Eudora Internet Mail ServerCRAM-MD5, DIGEST-MD5, PLAIN, LOGIN
EximCyrus SASL, Dovecot SASL, GNU SASL, CRAM-MD5, Heimdal GSSAPI, PLAIN, LOGIN, SPA
FirstClass
Gordano Messaging SuitePLAIN, LOGIN, CRAM-MD5, DIGEST-MD5, APOP, ODBC, Active Directory, NT Domain
GroupWiseeDirectory, Any LDAPv3-compliant source
HalonMany mechanism (SMTP callout, Dovecot SASL, REST, etc.) via script.[27]
HarakaProxy to other server, any other mechanism via AUTH plugins.
HCL Domino[28] Passkeys, Time-based one-time password
IceWarp Mail ServerPLAIN, LOGIN, CRAM-MD5, NTLM
Ipswitch IMail ServerActive Directory, Windows authentication, CRAM-MD5, PLAIN, LOGIN
KolabKolab employs Cyrus and Postfix and thus supports all Cyrus SASL authentication methods and X.509 PKI auth via STARTTLS and EXTERNAL.
MailsiteNT Domain, Active Directory, PLAIN, LOGIN, NTLM, SCRAM-MD5, CRAM-MD5
MailtraqNT Domain, Active Directory, Local Directory
MDaemon
Mercury Mail Transport SystemInternal, LOGIN, PLAIN, CRAM-MD5, Netware, LDAP?, partial NT Domain
Microsoft Exchange ServerActive Directory, LOGIN,[29] NTLM
NetMaileDirectory, LDAP
Oracle Communications Messaging ServerPlain, Login, CRAM-MD5, DIGEST-MD5, Certificate
OpenSMTPDBy default PLAIN and LOGIN over SSL only, Certificates, bsd_auth and PAM. Supports adding new authentication mechanisms through custom backends.
PostfixSupports all Cyrus SASL authentication methods except for APOP.
QpopperPluggable Authentication Modules, GSSAPI, CRAM-MD5, DIGEST-MD5, APOP, PLAIN, LDAP, CMU SASL library, Kerberos
qmailWorks with any checkpassword utility. The original supports only LOGIN and PLAIN. Alternate checkpassword utilities support authentication against PAM, LDAP, and databases.
ScalixInternal, LOGIN, PLAIN, X.500 (DAP), LDAP, OpenLDAP, Active Directory, eDirectory, Kerberos, etc.
SendmailSupports all Cyrus SASL authentication methods except for APOP. Also supports X.509 PKI auth via STARTTLS and EXTERNAL.
WinGateNT Domain, Active Directory, CRAM-MD5, SASL PLAIN, SASL LOGIN
ZarafaPLAIN, LOGIN using internal backends: Internal, Active Directory, any LDAP-compatible source, Unix
ZentyalActive Directory using Dovecot SASL mechanisms
ZimbraInternal, LDAP, Open LDAP, Active Directory
Mail serverSMTP AUTHPOP before SMTPAPOPFile SystemDatabaseLDAPNative Anti Hammer - Brute-force attack protectionOther_Authentication_possibilites

Antispam features

Mail ServerDNSBLSURBLSpamtrapsGreylistingSPFDKIMDMARCTarpitBayesian filtersRegular expressionsEmbedded AntivirusEmbedded Antispam
agorum core
Apache James ClamAV[30] (Apache SpamAssassin)
Axigen CYREN CYREN
Citadel (via SpamAssassin) (via SpamAssassin) (ClamAV) (SpamAssassin)
Courier Mail Server[31] [32] [33] Clamav
Dovecot[34]
Eudora Internet Mail Server
Exim (at build time, most distributions do) (at build time, most distributions do) (at build time, most distributions do) with spamassassin etc. any any
FirstClass Symantec
Gordano Messaging Suite CYREN, Authentium CYREN
GroupWise
Halon (via SpamAssassin) (Sophos, ClamAV) (CYREN, SpamAssassin)
Haraka with spamassassin etc. ClamAV, AVG, any via plugins SpamAssassin, MessageSniffer
HCL Domino[35] [36] [37] with ICAP
hMailServer ClamAV
IceWarp Mail Server
Ipswitch IMail Server[38] [39] Symantec, BitDefender CYREN
Kolab with spamassassin etc. (via content filter interface) (via content filter interface)
Mailsite
Mailtraq
MDaemon (via SpamAssassin) (via SpamAssassin) (via Content Filter) Cyren, ClamAV SpamAssassin
Mercury Mail Transport System (GreyWall extension) (SpamHalter extension) (ClamWall extension)
Microsoft Exchange Server (2003 & later) (Signing only via 3rd Party Extensions) (2003 & later)
NetMail
OpenSMTPD[40] [41]
Oracle Communications Messaging Server with spamassassin etc. any any
Postfix (via milter-link)[42] [43] with spamassassin etc. (via content filter interface) (via content filter interface)
qmailwith patchwith patch, with patch, with patch,
Scalix (via Scalix AntiSpam powered by Cyren or SpamAssassin) (via Scalix AntiSpam powered by Cyren or SpamAssassin) (via Scalix ZeroHour AntiVirus powered by Cyren or ClamAV)
Sendmail (via milter/SA) with spamassassin etc. ClamAV SpamAssassin
SparkEngine
UW IMAP
WinGate
Zarafa
Zimbra (via SpamAssassin) (via Cluebringer Policy Daemon) (partially) (via ClamAV, optionally any via Amavisd) (via SpamAssassin)
Mail ServerDNSBLSURBLSpamtrapsGreylistingSPFDKIMDMARCTarpitBayesian filtersRegular expressionsEmbedded AntivirusEmbedded Antispam

See also

Notes and References

  1. Web site: Michael S.. Mahoney. The Unix Oral History Project: Release.0, The Beginning. live. https://web.archive.org/web/20100119062927if_/http://www.princeton.edu/~hos/Mahoney/expotape.htm. 2010-01-19. en. mdy-all. To me one of the lovely features of pipes is the way it reinforces the notion of toolbox….
  2. Web site: 2013-08-19. COPYING. live. 2022-02-26. GitHub

    svarshavchik/courier-libs

    . en. mdy-all. This software is released under the GPL, version 3 (see COPYING.GPL).. https://web.archive.org/web/20220226043302/https://github.com/svarshavchik/courier-libs/blob/678e4860457b742c8d12aa820cabb3cf4b6024bd/common/COPYING . 2022-02-26 .
  3. Web site: RFCs Supported by Cyrus IMAP. 2021-04-27. Cyrus IMAP 3.4.3 (stable) documentation.
  4. Web site: Get Cyrus. Cyrus IMAP 3.4.3 (stable) documentation. 2022-02-25. https://web.archive.org/web/20170722061226if_/https://www.cyrusimap.org/imap/download/getcyrus.html#licensing. 2017-07-22. live. en. mdy-all.
  5. http://wiki.dovecot.org/OSCompatibility Dovecot Operating System Compatibility
  6. https://wiki2.dovecot.org/Submission Dovecot Submission Service
  7. Web site: Dovecot licensing information: COPYING. 2008-09-03.
  8. Web site: Exim 4.91 spec: Encrypted SMTP connections using TLS/SSL / 15. DANE. exim.org . 2018-07-05.
  9. See NOTICE file in the distribution
  10. Web site: Halon DANE support . Halon Security AB . 2015-12-17.
  11. Web site: Using HCL Traveler . 2022-11-17. HCL Traveler documentation.
  12. Web site: IMail v12.2 Release Notes. docs.ipswitch.com.
  13. Web site: Kopano Pricing - Subscription Plans - Kopano Partners.
  14. Web site: Microsoft Exchange Server 2007 RFC and Support for Standards: Exchange 2007 Help. Archiveddocs. technet.microsoft.com. 23 January 2017 .
  15. Web site: Exchange 2010 Support for RFC Standards: Exchange 2010 Help. Archiveddocs. technet.microsoft.com. 23 July 2014 .
  16. Web site: IPv6 Support in Exchange 2007 SP1 and SP2: Exchange 2007 Help. Archiveddocs. technet.microsoft.com. 23 January 2017 .
  17. Web site: OpenSMTPD, current state of affairs: Lookup layer change . OpenBSD.org . 2018-07-24.
  18. Web site: Licenses-OX-APP-Suite - Open-Xchange. www.open-xchange.com. 2016-02-24. 2016-03-03. https://web.archive.org/web/20160303023145/https://www.open-xchange.com/en/licenses-ox-app-suite. dead.
  19. Contains jurisdiction selection and IPL-style indemnification clause.
  20. https://fossies.org/linux/sendmail/RELEASE_NOTES Sendmail Release Notes
  21. Web site: UW IMAP software--UW IMAP Server Documentation. www.washington.edu.
  22. Web site: Licenses and Terms of Use. www.zimbra.com. live. https://web.archive.org/web/20140901211205if_/http://www.zimbra.com/legal/licensing. 2014-09-01. 2022-02-25. en. mdy-all.
  23. https://tools.ietf.org/html/draft-ietf-sasl-crammd5-to-historic-00 CRAM-MD5 to Historic, 20 November 2008
  24. https://tools.ietf.org/html/rfc6331 Moving DIGEST-MD5 to Historic, July 2011
  25. Web site: DovecotFeatures - Dovecot Wiki. wiki.dovecot.org.
  26. Web site: Authentication - Dovecot Wiki. wiki.dovecot.org.
  27. Web site: HSL - Halon SMTP server. Halon Security AB . 2016-02-11 .
  28. Web site: HCL Domino Documentation - Using Internet password lockout. 30 June 2024. help.hcl-software.com.
  29. Web site: How to Configure Exchange to Allow AUTH LOGIN to work. support2.gwava.com.
  30. Web site: Documentation for built-in ClamAV-Mailet . 2013-03-19.
  31. Web site: Index of /~gordon/courier-pythonfilter. phantom.dragonsdawn.net.
  32. Two DKIM filters can be found in Courier-MTA's links page
  33. With Web site: zdkimfilter.
  34. Considered and rejected by developers in 2013. Web site: Fawcett. John. 2013-07-03. [Dovecot] dnsbl feature for dovecot. live. https://web.archive.org/web/20220225204208if_/https://dovecot.org/list/dovecot/2013-July/091236.html. 2022-02-25. 2022-02-25. dovecot.org. en. mdy-all.
  35. Web site: HCL Domino Documentation - Restricting SMTP inbound routing. 30 June 2024. help.hcl-software.com.
  36. Web site: Configuring the Domino SMTP server to reduce spam. 5 June 2024. help.hcl-software.com.
  37. Web site: Configuring DKIM signing for messages routed to external Internet domains. 5 June 2024. help.hcl-software.com.
  38. Web site: IMail Secure Mail Server - Ipswitch. Ipswitch. Inc.. www.imailserver.com.
  39. Web site: Your IMail Software Upgrades and Patches - Ipswitch IMail Server. Ipswitch. Inc.. www.imailserver.com.
  40. With Web site: filter-dkimsign. or Web site: filter-rspamd. . 15 October 2022 .
  41. With Web site: filter-rspamd. . 15 October 2022 .
  42. Web site: OpenDKIM. www.opendkim.org.
  43. Web site: Trusted Domain Project - About Us. www.trusteddomain.org.