# 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 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

| | OS support | OS support | OS support | Protocol support | Protocol support | Protocol support | Protocol support | Protocol support | Protocol support | Protocol support | Features | Features | Features | Features | Features | Features | Features | Storage | 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 | License |
| 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+ |
| Cyrus IMAP | Yes | No | Yes | No | Yes | Yes | Yes | Yes | No | Yes | Yes | Yes | No | No | No | Yes | No | Indexes only | Yes | BSD 4-clause |
| DBMail | Yes | No | Yes | No | Yes | Yes | No | Yes | No | Yes | Yes | Yes | No | No | No | No | Yes | Yes | No | GPLv2 |
| Dovecot | Yes | No | Yes | Yes | Yes | Yes | No | Yes | Yes | Yes | Yes | Yes | No | No | No | Yes | No | Yes | maildir, mbox, dbox, mdbox, obox | Mixed: MIT and LGPL-2.1 |
| 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 | No | No | Yes | Yes | Yes | Yes | GPLv2+ |
| 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 | 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 | 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) | 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. | Yes | Yes | No | Yes | postfix | Yes | Yes | Yes | postfix | Yes | z-push | No | Yes | SQL | Yes | AGPLv3, paid |
| 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 | Yes | Yes | Yes | 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 | 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 |
| 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) |
| 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) | 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 |
| WinGate | No | Yes | No | Yes | Yes | Yes | ? | Yes | Yes | Yes | No | Yes | ? | Yes | No | ? | ? | No | Yes | Proprietary |
| Zarafa | Yes | No | No | postfix etc. | Yes | Yes | No | Yes | postfix | Yes | Yes | Yes | postfix | Yes | z-push | No | No | SQL | Yes | AGPLv3, paid |
| 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 |
| 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

| 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, NTLM |
| Apache James | Yes | ? | ? | ? | Yes | Yes | ? | PLAIN, LOGIN |
| Axigen | Yes | No | Yes | Yes | No | Yes | No | GSSAPI, CRAM-MD5, DIGEST-MD5, APOP, CLRTXT, LOGIN, PLAIN, LDAP, Internal |
| CommuniGate Pro | Yes | Yes | Yes | Yes | Yes | Yes | ? | Active 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 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, 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 |
| DBMail | No | No | No | No | Yes | Yes | No | LOGIN, Active Directory, OpenLDAP |
| Dovecot | Yes | Yes | Yes | Yes | Yes | Yes | ? | Submission service supports SMTP AUTH. Offers SASL to MTAs. 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 Server | Yes | No | Yes | Yes | No | No | Yes | CRAM-MD5, DIGEST-MD5, PLAIN, LOGIN |
| Exim | Yes | Yes | Yes | Yes | Yes | Yes | ? | Cyrus SASL, Dovecot SASL, GNU SASL, CRAM-MD5, Heimdal GSSAPI, PLAIN, LOGIN, SPA |
| FirstClass | Yes | ? | ? | Yes | Yes | Yes | ? | |
| Gordano Messaging Suite | Yes | Yes | Yes | No | Yes | Yes | ? | PLAIN, LOGIN, CRAM-MD5, DIGEST-MD5, 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. |
| 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 | Passkeys, Time-based one-time password |
| IceWarp Mail Server | Yes | Yes | Yes | Yes | Yes | Yes | ? | PLAIN, LOGIN, CRAM-MD5, NTLM |
| Ipswitch IMail Server | Yes | No | Yes | Yes | Yes | Yes | Yes | Active Directory, Windows authentication, CRAM-MD5, 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 |
| 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, Netware, LDAP?, partial NT Domain |
| Microsoft Exchange Server | Yes | ? | Yes | No | No | Yes | Yes | Active Directory, LOGIN, NTLM |
| NetMail | Yes | Yes | Yes | Yes | Yes | Yes | ? | eDirectory, LDAP |
| Oracle Communications Messaging Server | Yes | Yes | Yes | Yes | Yes | Yes | ? | Plain, Login, CRAM-MD5, DIGEST-MD5, 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, DIGEST-MD5, 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, 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

| 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 | Optional (Apache SpamAssassin) |
| Axigen | Yes | No | Yes | Yes | Yes | Yes | No | 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 | Yes | Optional | Optional | Yes | Yes | Yes | Yes Clamav | Yes |
| Dovecot | No | 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 | ? | ? | ? | Yes | Yes | ? | ? | ? | ? | 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 | Yes | 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 | Optional | ? | ? | ? | ? | ? |
| 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 | Optional | 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 (via Scalix AntiSpam powered by Cyren or SpamAssassin) | Yes (via Scalix AntiSpam powered by Cyren or SpamAssassin) | Yes (via Scalix AntiSpam powered by Cyren or SpamAssassin) | Yes | Yes | Yes | Yes (via Scalix ZeroHour AntiVirus powered by Cyren or ClamAV) | Yes |
| Sendmail | Yes | Yes (via milter/SA) | ? | Yes | Optional | Optional | Optional | ? | Optional with spamassassin etc. | Yes | Yes ClamAV | Yes SpamAssassin |
| SparkEngine | ? | ? | ? | ? | ? | ? | ? | ? | ? | ? | ? | ? |
| UW IMAP | ? | ? | ? | ? | ? | ? | ? | ? | ? | ? | ? | ? |
| WinGate | ? | ? | ? | ? | ? | ? | ? | ? | ? | ? | ? | ? |
| Zarafa | External | External | External | External | External | External | External | External | External | External | External | External |
| 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 |