Wikipedia

Comparison of mail servers

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

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OS support Protocol support Features Storage License
Mail server Linux/Unix Windows macOS SMTP POP3 IMAP JMAP IDLE SMTPS POP3S IPv6 SSL DANE Webmail ActiveSync Sieve IDN/UTF8 Database File-based
agorum core Yes Yes No Yes No Yes No No No No No Yes No Yes No No ? Yes No GPLv2
Apache James Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes ? Yes ? No No Yes ? Yes Yes ASLv2
Axigen Yes Yes No Yes Yes Yes ? Yes Yes Yes Yes Yes ? Ajax, Basic & Mobile Yes Yes Yes Embedded, Optimized for Email & Groupware Yes Proprietary (free version: 5 domains, 5 users, 5 groups)
Citadel Yes No Yes Yes Yes Yes ? No Yes Yes Yes Yes ? Yes No Yes ? Yes No GPLv3
CommuniGate Pro Yes Yes Yes Yes Yes Yes ? Yes Yes Yes Yes Yes ? Yes Yes ? Yes Yes Yes Proprietary (Free for up to 5 users)
Courier Mail Server Yes No Yes Yes Yes Yes ? Yes Yes Yes Yes Yes No Yes No maildrop Yes No maildir GPLv3+[2]
Cyrus IMAP Yes No Yes No Yes Yes Yes Yes No Yes Yes Yes No No No Yes No[3] Indexes only Yes BSD 4-clause[4]
DBMail Yes No Yes No Yes Yes No Yes No Yes Yes Yes No No No No Yes Yes No GPLv2
Dovecot Yes[5] No Yes Yes[6] Yes Yes No Yes Yes Yes Yes Yes No No No Yes No Yes maildir, mbox, dbox, mdbox, obox[7] Mixed: MIT and LGPL-2.1[8]
Eudora Internet Mail Server No No Yes Yes Yes Yes ? ? No No ? No ? No No ? ? No No Proprietary
Exim Yes Yes (via Cygwin) Yes Yes No No ? No Yes No Yes Yes Yes[9] No No Yes Yes Yes Yes GPLv2+[10]
FirstClass Yes Yes Yes Yes Yes Yes ? ? Yes Yes ? Yes ? Yes Yes ? ? Yes Yes Proprietary
Gordano Messaging Suite Yes Yes No Yes Yes Yes ? Yes Yes Yes ? Yes ? Yes Yes No ? Yes Yes Proprietary
GroupWise Yes Yes No Yes Yes Yes ? ? Yes Yes ? Yes ? Yes Yes ? ? Yes No Proprietary
Halon Yes No No Yes No No ? No Yes No Yes Yes Yes[11] No No ? Yes Yes Yes Proprietary
Haraka Yes Yes Yes Yes No No ? No Yes No Yes Yes ? No No ? Yes Yes Yes MIT
hMailServer No Yes No Yes Yes Yes ? Yes Yes Yes Yes Yes ? Yes No ? No Yes Yes GNU AGPL
HCL Domino Yes Yes No Yes Yes Yes No No Yes Yes Yes Yes No Yes Yes [12] No Yes Yes No Proprietary
IceWarp Mail Server Yes Yes Yes Yes Yes Yes ? Yes Yes Yes Yes Yes No Yes Yes No ? Yes Yes Proprietary
Ipswitch IMail Server No Yes No Yes Yes Yes ? Yes (since v12.2)[13] Yes Yes ? Yes ? Yes Yes ? ? No Yes Proprietary
Kerio Connect Yes Yes Yes Yes Yes Yes ? Yes Yes Yes Yes Yes ? Yes Yes Yes ? No Yes Proprietary
Kolab Yes No No Yes Yes Yes ? Yes Yes Yes Yes Yes Yes Yes Yes Yes ? Yes Yes GPLv3
Kopano Yes No No postfix, etc.[14] Yes Yes No Yes postfix Yes Yes Yes postfix Yes z-push No Yes SQL Yes AGPLv3, paid[15]
KumoMTA Yes No No Yes No No ? No Yes No Yes Yes Yes No No Lua Yes Yes Yes ASLv2
Mailsite No Yes No Yes Yes Yes ? ? Yes Yes ? Yes ? Yes Yes Yes ? Yes Yes Proprietary
Mailtraq No Yes No Yes Yes Yes ? Yes Yes Yes ? Yes ? Yes No ? ? Yes Yes Proprietary
MDaemon No Yes No Yes Yes Yes ? Yes Yes Yes Yes Yes No Yes Yes No Not IMAP No Yes Proprietary
Mercury Mail Transport System No Yes No Yes Yes Yes ? Yes Yes Yes No Yes No Yes No No No No Yes Proprietary donationware
Microsoft Exchange Server No Yes No Yes Yes Yes No Yes[16][17] Yes Yes Yes [18] Yes No Yes Yes 3rd Party support Not IMAP Yes Yes Proprietary
NetMail Yes Yes No Yes Yes Yes ? ? Yes Yes ? Yes ? Yes No ? ? Yes No Proprietary
OpenSMTPD Yes No Yes Yes No No No No Yes No Yes Yes No[19] No No No No via custom backends Yes ISC
Open-Xchange Yes No No Yes Yes Yes ? ? Yes Yes ? Yes ? Yes Yes ? ? Yes No Proprietary/SaaS, with open-source parts[20]
Oracle Communications Messaging Server Yes No No Yes Yes Yes ? ? Yes Yes ? Yes ? Yes No ? Yes Yes Yes Proprietary
Postfix Yes No Yes Yes No No ? No Yes No Yes Yes Yes No No No Yes Yes Yes IBM Public License
qmail Yes No Yes Yes Yes No No No with ucspi-ssl, with patch with ucspi-ssl s/qmail, via Qsmtp with ucspi-ssl, with patch s/qmail No No No s/qmail (cdb) Yes Public domain
Qpopper Yes Yes (via Cygwin) Yes No Yes No ? No No Yes ? Yes ? No No ? ? No Yes BSD-style (GPL-incompatible)[21]
Scalix Yes No No Yes Yes Yes ? Yes Yes (via Stunnel) Yes (via Stunnel) ? Yes (via Stunnel) ? Yes Yes ? ? Yes Yes Proprietary with OSS parts, a community (Freeware) and 3 paid editions (SMB, Enterprise, Hosting)
Sendmail Yes No Yes Yes No No ? No Yes No Yes Yes Yes (since 8.16)[22] No No No Yes ? Yes Sendmail License
SparkEngine Yes Yes Yes Yes No No ? No Yes No ? Yes ? No No ? ? Yes Yes Proprietary
UW IMAP Yes Yes Yes No Yes Yes ? Yes No Yes Yes Yes No WebPine No No ? Yes (Indexes) Yes Apache License 2.0[23]
WinGate No Yes No Yes Yes Yes ? Yes Yes Yes No Yes ? Yes No ? ? No Yes Proprietary
Zarafa Yes No No postfix etc.[14] Yes Yes No Yes postfix Yes Yes Yes postfix Yes z-push No No SQL Yes AGPLv3, paid[15]
Zentyal Yes No No Yes Yes Yes ? Yes Yes Yes No Yes ? Yes Yes ? ? Yes Yes Proprietary, 45-day trial
Zimbra Yes No Yes Yes Yes Yes ? Yes Yes Yes Yes Yes Yes Yes Yes Yes No Yes Yes ZPL and proprietary editions[24]
Mail server Linux/Unix Windows macOS SMTP POP3 IMAP JMAP IDLE SMTPS POP3S IPv6 SSL DANE Webmail ActiveSync Sieve IDN/UTF8 Database File-based License

Authentication

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Mail server SMTP AUTH POP before SMTP APOP File System Database LDAP Native Anti Hammer - Brute-force attack protection Other
agorum core Yes No No No Yes Yes ? PLAIN, LOGIN, CRAM-MD5,[25] NTLM
Apache James Yes ? ? ? Yes Yes ? PLAIN, LOGIN
Axigen Yes No Yes Yes No Yes No GSSAPI, CRAM-MD5,[25] DIGEST-MD5,[26] APOP, CLRTXT, LOGIN, PLAIN, LDAP, Internal
CommuniGate Pro Yes Yes Yes Yes Yes Yes ? Active Directory (LDAP), CLRTXT, CRAM-MD5,[25] DIGEST-MD5,[26] APOP, GSSAPI, NTLM, MSN, SESSIONID, External Authentication Plug-in. Also supports X.509 PKI auth via STARTTLS/SSL
Courier Mail Server Yes Yes Yes Yes Yes Yes ? Managed by Courier authentication library which can use PAM, Userdb, PostgreSQL(beta), MySQL, LDAP, and EXTERNAL.
Cyrus IMAP Yes Yes Yes Yes Yes Yes ? Supports 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,[25] DIGEST-MD5,[26] 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
DBMail No No No No Yes Yes No LOGIN, Active Directory, OpenLDAP
Dovecot Yes Yes Yes Yes Yes Yes ? Submission service supports SMTP AUTH.[6] Offers SASL to MTAs.[27][28] 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,[25] DIGEST-MD5,[26] SCRAM-SHA-1, EXTERNAL, GSSAPI, NTLM, OTP, SKEY, RPA.
Eudora Internet Mail Server Yes No Yes Yes No No Yes CRAM-MD5,[25] DIGEST-MD5,[26] PLAIN, LOGIN
Exim Yes Yes Yes Yes Yes Yes ? Cyrus SASL, Dovecot SASL, GNU SASL, CRAM-MD5,[25] Heimdal GSSAPI, PLAIN, LOGIN, SPA
FirstClass Yes ? ? Yes Yes Yes ?
Gordano Messaging Suite Yes Yes Yes No Yes Yes ? PLAIN, LOGIN, CRAM-MD5,[25] DIGEST-MD5,[26] APOP, ODBC, Active Directory, NT Domain
GroupWise Yes Yes Yes Yes Yes Yes ? eDirectory, Any LDAPv3-compliant source
Halon Yes No No Yes Yes Yes ? Many mechanism (SMTP callout, Dovecot SASL, REST, etc.) via script.[29]
Haraka Yes No No Yes Yes Yes ? Proxy to other server, any other mechanism via AUTH plugins.
HCL Domino Yes No No No Yes Yes Yes[30] Passkeys, Time-based one-time password
IceWarp Mail Server Yes Yes Yes Yes Yes Yes ? PLAIN, LOGIN, CRAM-MD5,[25] NTLM
Ipswitch IMail Server Yes No Yes Yes Yes Yes Yes Active Directory, Windows authentication, CRAM-MD5,[25] PLAIN, LOGIN
Kolab Yes Yes Yes Yes Yes Yes ? Kolab employs Cyrus and Postfix and thus supports all Cyrus SASL authentication methods and X.509 PKI auth via STARTTLS and EXTERNAL.
Mailsite Yes No Yes Yes Yes Yes ? NT Domain, Active Directory, PLAIN, LOGIN, NTLM, SCRAM-MD5, CRAM-MD5[25]
Mailtraq Yes Yes Yes Yes Yes Yes ? NT Domain, Active Directory, Local Directory
MDaemon Yes No Yes No No Yes Yes
Mercury Mail Transport System Yes Yes Yes Yes No ? ? Internal, LOGIN, PLAIN, CRAM-MD5,[25] Netware, LDAP?, partial NT Domain
Microsoft Exchange Server Yes ? Yes No No Yes Yes Active Directory, LOGIN,[31] NTLM
NetMail Yes Yes Yes Yes Yes Yes ? eDirectory, LDAP
Oracle Communications Messaging Server Yes Yes Yes Yes Yes Yes ? Plain, Login, CRAM-MD5,[25] DIGEST-MD5,[26] Certificate
OpenSMTPD Yes No No Yes Yes Yes No By default PLAIN and LOGIN over SSL only, Certificates, bsd_auth and PAM. Supports adding new authentication mechanisms through custom backends.
Postfix Yes No No Yes Yes Yes Yes Supports all Cyrus SASL authentication methods except for APOP.
Qpopper No Yes Yes Yes Yes Yes ? Pluggable Authentication Modules, GSSAPI, CRAM-MD5,[25] DIGEST-MD5,[26] APOP, PLAIN, LDAP, CMU SASL library, Kerberos
qmail with patch, s/qmail with vpopmail Yes Yes Yes Yes ? Works with any checkpassword utility. The original supports only LOGIN and PLAIN. Alternate checkpassword utilities support authentication against PAM, LDAP, and databases.
Scalix Yes ? Yes Yes Yes Yes ? Internal, LOGIN, PLAIN, X.500 (DAP), LDAP, OpenLDAP, Active Directory, eDirectory, Kerberos, etc.
Sendmail Yes Yes No Yes Yes Yes ? Supports all Cyrus SASL authentication methods except for APOP. Also supports X.509 PKI auth via STARTTLS and EXTERNAL.
WinGate Yes Yes Yes Yes Yes No ? NT Domain, Active Directory, CRAM-MD5,[25] SASL PLAIN, SASL LOGIN
Zarafa Yes No No Yes Yes Yes ? PLAIN, LOGIN using internal backends: Internal, Active Directory, any LDAP-compatible source, Unix
Zentyal Yes Yes Yes Yes Yes Yes ? Active Directory using Dovecot SASL mechanisms
Zimbra Yes No Yes Yes Yes Yes ? Internal, LDAP, Open LDAP, Active Directory
Mail server SMTP AUTH POP before SMTP APOP File System Database LDAP Native Anti Hammer - Brute-force attack protection Other_Authentication_possibilites

Antispam features

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Mail Server DNSBL SURBL Spamtraps Greylisting SPF DKIM DMARC Tarpit Bayesian filters Regular expressions Embedded Antivirus Embedded Antispam
agorum core No No No No No No No No No Yes No No
Apache James ? ? ? ? ? ? ? ? Yes ? Yes ClamAV[32] Optional (Apache SpamAssassin)
Axigen Yes No Yes Yes Yes Yes ? No Yes, in SpamAssassin Yes Yes CYREN Yes CYREN
Citadel Yes Yes No No Optional (via SpamAssassin) ? ? No Optional (via SpamAssassin) No Optional (ClamAV) Optional (SpamAssassin)
Courier Mail Server Yes ? Yes Yes[33] Yes Optional[34] Optional[35] Yes Yes[33] Yes[33] Yes Clamav Yes[33]
Dovecot No[36] No ? ? ? ? ? ? ? ? ? ?
Eudora Internet Mail Server Yes ? Yes Yes ? ? ? ? ? ? ? ?
Exim Yes Yes Yes Yes Optional (at build time, most distributions do) Optional (at build time, most distributions do) Optional (at build time, most distributions do) Yes Optional with spamassassin etc. Yes Optional any Optional any
FirstClass Yes Yes No ? Yes ? ? Yes Yes Yes Yes Symantec ?
Gordano Messaging Suite Yes Yes Yes Yes Yes ? ? Yes Yes Yes Yes CYREN, Authentium Yes CYREN
GroupWise Yes ? ? ? ? ? ? ? ? ? ? ?
Halon Yes Yes Yes Yes Yes Yes Yes Yes Optional (via SpamAssassin) Yes Optional (Sophos, ClamAV) Optional (CYREN, SpamAssassin)
Haraka Yes Yes Yes Yes Yes Yes ? Yes Optional with spamassassin etc. Yes Optional ClamAV, AVG, any via plugins Optional SpamAssassin, MessageSniffer
HCL Domino Yes[37] ? ? ? Yes[38] Yes[39] ? ? ? ? Optional with ICAP ?
hMailServer Yes Yes No Yes Yes Yes ? No No Yes Yes ClamAV Yes SpamAssassin
IceWarp Mail Server Yes Yes Yes Yes Yes ? ? Yes Yes Yes Yes Yes
Ipswitch IMail Server Yes Yes No No Yes Yes[40] Yes[41] No Yes Yes Yes Symantec, BitDefender Yes CYREN
Kolab Yes ? ? Yes Yes ? ? ? Optional with spamassassin etc. Yes Optional (via content filter interface) Optional (via content filter interface)
Mailsite Yes Yes Yes Yes Yes ? ? No Yes Yes Yes Yes
Mailtraq ? ? ? ? ? ? ? ? ? ? ? ?
MDaemon Yes Yes (via SpamAssassin) Yes Yes Yes Yes Yes Yes Yes (via SpamAssassin) Yes (via Content Filter) Optional Cyren, ClamAV Yes SpamAssassin
Mercury Mail Transport System Yes ? ? Yes (GreyWall extension) ? ? ? ? Yes (SpamHalter extension) ? Yes (ClamWall extension) ?
Microsoft Exchange Server Yes (2003 & later) ? ? Yes Yes Optional (Signing only via 3rd Party Extensions) ? Yes (2003 & later) ? ? ? ?
NetMail ? ? ? ? ? ? ? ? ? ? ? ?
OpenSMTPD ? ? ? ? Yes Optional[42] Optional[43] ? ? ? ? ?
Oracle Communications Messaging Server Yes Yes Yes Yes Yes ? ? Yes Optional with spamassassin etc. Yes Optional any Optional any
Postfix Yes Yes (via milter-link) ? Yes Yes Optional[44] Optional[45] Yes Optional with spamassassin etc. Yes Optional (via content filter interface) Optional (via content filter interface)
qmail Yes with patch ? Yes with patch, s/qmail with patch, s/qmail ? with patch, s/qmail No ? No No
Scalix Yes Yes (via Scalix AntiSpam powered by Cyren or SpamAssassin) Yes (via Scalix AntiSpam powered by Cyren or SpamAssassin) Yes Yes Yes Yes Yes Yes Yes Yes (via Scalix ZeroHour AntiVirus powered by Cyren or ClamAV) Yes
Sendmail Yes Yes (via milter/SA) ? Yes Optional Optional[44] Optional[45] ? Optional with spamassassin etc. Yes Yes ClamAV Yes SpamAssassin
SparkEngine ? ? ? ? ? ? ? ? ? ? ? ?
UW IMAP ? ? ? ? ? ? ? ? ? ? ? ?
WinGate ? ? ? ? ? ? ? ? ? ? ? ?
Zarafa External[14] External[14] External[14] External[14] External[14] External[14] External[14] External[14] External[14] External[14] External[14] External[14]
Zimbra Yes Yes (via SpamAssassin) ? Yes (via Cluebringer Policy Daemon) Yes Yes ? Yes Yes Yes (partially) Yes (via ClamAV, optionally any via Amavisd) Yes (via SpamAssassin)
Mail Server DNSBL SURBL Spamtraps Greylisting SPF DKIM DMARC Tarpit Bayesian filters Regular expressions Embedded Antivirus Embedded Antispam

See also

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References

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  1. ^ Mahoney, Michael S. "The Unix Oral History Project: Release.0, The Beginning". Archived from the original on January 19, 2010. To me one of the lovely features of pipes is the way it reinforces the notion of toolbox…
  2. ^ "COPYING". GitHub: svarshavchik/courier-libs. August 19, 2013. Archived from the original on February 26, 2022. Retrieved February 26, 2022. This software is released under the GPL, version 3 (see COPYING.GPL).
  3. ^ "RFCs Supported by Cyrus IMAP". Cyrus IMAP 3.4.3 (stable) documentation. Retrieved 2021-04-27.
  4. ^ "Get Cyrus". Cyrus IMAP 3.4.3 (stable) documentation. Archived from the original on July 22, 2017. Retrieved February 25, 2022.
  5. ^ Dovecot Operating System Compatibility. Retrieved on 2008-09-03.
  6. ^ a b Dovecot Submission Service. Retrieved on 2010-07-15.
  7. ^ "Dovecot Object Storage support". Retrieved 2021-01-25.
  8. ^ "Dovecot licensing information: COPYING". Retrieved 2008-09-03.
  9. ^ "Exim 4.91 spec: Encrypted SMTP connections using TLS/SSL / 15. DANE". exim.org. Retrieved 2018-07-05.
  10. ^ See NOTICE file in the distribution
  11. ^ "Halon DANE support". Halon Security AB. Retrieved 2015-12-17.[self-published source]
  12. ^ "Using HCL Traveler". HCL Traveler documentation. Retrieved 2022-11-17.
  13. ^ "IMail v12.2 Release Notes". docs.ipswitch.com.
  14. ^ a b c d e f g h i j k l m n Zarafa Collaboration Platform architecture diagram. Retrieved on 2011-12-10.
  15. ^ a b "Kopano Pricing - Subscription Plans - Kopano Partners".
  16. ^ Archiveddocs (23 January 2017). "Microsoft Exchange Server 2007 RFC and Support for Standards: Exchange 2007 Help". technet.microsoft.com.
  17. ^ Archiveddocs (23 July 2014). "Exchange 2010 Support for RFC Standards: Exchange 2010 Help". technet.microsoft.com.
  18. ^ Archiveddocs (23 January 2017). "IPv6 Support in Exchange 2007 SP1 and SP2: Exchange 2007 Help". technet.microsoft.com.
  19. ^ "OpenSMTPD, current state of affairs: Lookup layer change" (PDF). OpenBSD.org. Retrieved 2018-07-24.[self-published source]
  20. ^ "Licenses-OX-APP-Suite - Open-Xchange". www.open-xchange.com. Archived from the original on 2016-03-03. Retrieved 2016-02-24.
  21. ^ Contains jurisdiction selection and IPL-style indemnification clause.
  22. ^ Sendmail Release Notes. Retrieved on 2021-01-08.
  23. ^ "UW IMAP software--UW IMAP Server Documentation". www.washington.edu.
  24. ^ "Licenses and Terms of Use". www.zimbra.com. Archived from the original on September 1, 2014. Retrieved February 25, 2022.
  25. ^ a b c d e f g h i j k l m n o CRAM-MD5 to Historic, 20 November 2008.
  26. ^ a b c d e f g h Moving DIGEST-MD5 to Historic, July 2011.
  27. ^ "DovecotFeatures - Dovecot Wiki". wiki.dovecot.org.
  28. ^ "Authentication - Dovecot Wiki". wiki.dovecot.org.
  29. ^ "HSL - Halon SMTP server". Halon Security AB. Retrieved 2016-02-11.
  30. ^ "HCL Domino Documentation - Using Internet password lockout". help.hcl-software.com. 30 June 2024.
  31. ^ "How to Configure Exchange to Allow AUTH LOGIN to work". support2.gwava.com.
  32. ^ "Documentation for built-in ClamAV-Mailet". Retrieved 2013-03-19.
  33. ^ a b c d "Index of /~gordon/courier-pythonfilter". phantom.dragonsdawn.net.
  34. ^ Two DKIM filters can be found in Courier-MTA's links page
  35. ^ With "zdkimfilter".
  36. ^ Considered and rejected by developers in 2013. Fawcett, John (July 3, 2013). "[Dovecot] dnsbl feature for dovecot". dovecot.org. Archived from the original on February 25, 2022. Retrieved February 25, 2022.
  37. ^ "HCL Domino Documentation - Restricting SMTP inbound routing". help.hcl-software.com. 30 June 2024.
  38. ^ "Configuring the Domino SMTP server to reduce spam". help.hcl-software.com. 5 June 2024.
  39. ^ "Configuring DKIM signing for messages routed to external Internet domains". help.hcl-software.com. 5 June 2024.
  40. ^ Inc., Ipswitch. "IMail Secure Mail Server - Ipswitch". www.imailserver.com. {{cite web}}: |last= has generic name (help)
  41. ^ Inc., Ipswitch. "Your IMail Software Upgrades and Patches - Ipswitch IMail Server". www.imailserver.com. {{cite web}}: |last= has generic name (help)
  42. ^ With "filter-dkimsign". or "filter-rspamd". GitHub. 15 October 2022.
  43. ^ With "filter-rspamd". GitHub. 15 October 2022.
  44. ^ a b "OpenDKIM". www.opendkim.org.
  45. ^ a b "Trusted Domain Project - About Us". www.trusteddomain.org.
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