hMailServer FAQ

Yes, hMailServer is still maintained. Progressive Robot Ltd forked it in 2026 and maintains the hMailServer 6 line. The current release is hMailServer 6.3.3, released on 15 September 2026 for Windows and Linux.

Martin Knafve created hMailServer. The original project went quiet for years. In 2026 Progressive Robot forked it, rebuilt it on a current toolchain (Visual Studio 2026 and OpenSSL 4, 64-bit only) and released hMailServer 6.0.0 on 12 June 2026. Linux support for x86-64 and AArch64 arrived in hMailServer 6.3.0 on 10 September 2026. The original project has since resumed development as well. Its changes are reviewed against the hMailServer 6 code, and several have been ported.

Release notes, SHA-256 checksums and packages for every version are on the downloads page. New releases are announced in Announcements & releases.

Progressive Robot Ltd, based at Chester Business Park in Chester, United Kingdom, forked hMailServer in 2026 and maintains the hMailServer 6 line. hMailServer was originally created by Martin Knafve, and the original project has since resumed development independently of this fork.

Progressive Robot has published hMailServer 6 releases since 6.0.0 on 12 June 2026; the current release is hMailServer 6.3.3, released 15 September 2026. Since 6.3.1 the Windows installer has carried an Authenticode signature naming Progressive Robot Ltd, and the 6.3.2 release notes state that the project's GOVERNANCE.md names two maintainers.

Progressive Robot also runs this forum, where its staff post as chris and Progressiverobot. hMailServer remains free and open source under AGPL-3.0, with no per-mailbox fees and no paid tiers. See About this forum, the official hMailServer page and the GitLab repository.

hMailServer was originally created by Martin Knafve. The readme of hMailServer 6.0.0 Beta 1, dated 12 June 2026, credits the software as "Based on hMailServer by Martin Knafve". Under the original project it was a free, Windows-only mail server from 2002.

Progressive Robot did not create hMailServer. It forked the original project in 2026, after that project had gone quiet for years, and maintains the hMailServer 6 line. The first production release, hMailServer 6.0.0, is dated 12 June 2026, and hMailServer 6.3.0 added Linux support on 10 September 2026. The original project has since resumed development. The notes for the hMailServer 6.2.23-alpha2 pre-release of 4 September 2026 record that seven fixes were taken from it. See About this forum.

hMailServer is free and open source under the AGPL-3.0 licence, including for commercial use, with no per-mailbox fees and no paid tiers. The current release is hMailServer 6.3.3, released on 15 September 2026. It is a free download for Windows (x64 only) and for Linux on x86-64 and AArch64. Simply using hMailServer requires nothing of you under the licence. If you modify it and offer it to others over a network, you must publish your changes.

The Windows installer and the Linux .deb and .rpm packages are on the official downloads page, and the source code is on GitLab. Progressive Robot Ltd, which maintains the hMailServer 6 line, sells optional managed hMailServer services and support plans. No answer on this forum is gated behind a support plan.

hMailServer 6.3.3 is the latest release of hMailServer, published on 15 September 2026 for Windows and Linux under the AGPL-3.0 licence. It ships as seven signed packages: a 64-bit Windows installer, plus a .deb, an .rpm and an AppImage for both x86-64 and AArch64. The AppImage is for trying hMailServer out and cannot bind port 25; use the .deb or .rpm to run mail.

hMailServer 6.3.3 fixes outbound BDAT: since outbound BDAT arrived in 6.2.25, any message over 60,000 bytes sent to a server advertising CHUNKING stalled. It also rebuilds the webmail and adds CardDAV for the address book. Both are off by default: the webmail needs RestApiPort and CardDAV needs WebServicesHttpsPort set in hMailServer.ini.

If you are upgrading from anything older than 6.3.1, go straight to 6.3.3, because 6.3.2 could not upgrade an older database schema. If you run 6.3.1 or 6.3.2 with the Control Panel's live update, update by hand once: close the Control Panel, then run the 6.3.3 installer. Releases are posted on the downloads page and in Announcements & releases.

Yes, hMailServer 5.3, 5.4, 5.5, 5.6 and 5.7 installations on Windows upgrade in place directly to hMailServer 6.3.3, released on 15 September 2026, with no intermediate versions. Run hMailServer-6.3.3-x64.exe as administrator and accept the upgrade. The installer upgrades the database schema and keeps mail, accounts, domains and settings.

Do not use hMailServer 6.3.2: it stopped every upgrade from 5.x with "The server has not loaded its configuration". The schema upgrade is one-way, so back up the database, the data directory and hMailServer.INI first, and on 5.3 to 5.5 also test the restore. On a large database, plan a maintenance window for two of the schema steps.

The classic Administrator (hMailAdmin.exe) is replaced by the Control Panel (hMailCP.exe), and the PHP WebAdmin is gone. If the installer cannot add the .NET 10 Desktop Runtime, the schema is not upgraded and the service will not start: install the runtime by hand, then run DBUpdater.exe. See chapter 18 of the hMailServer documentation or ask in Installation, migration & upgrades.

To verify an hMailServer download, compare its SHA-256 hash with the value on the official downloads page, or check it with cosign against the Sigstore .cosign.bundle published beside it. In hMailServer 6.3.3, the current release (15 September 2026), every installer, package, checksum list and SBOM has a bundle.

On Windows, run Get-FileHash .\hMailServer-6.3.3-x64.exe -Algorithm SHA256 in PowerShell. On Linux, run sha256sum -c hmailserver-linux-6.3.3-SHA256SUMS.txt --ignore-missing. With cosign, run cosign verify-blob --bundle hMailServer-6.3.3-x64.exe.cosign.bundle --certificate-identity-regexp '^https://github\.com/Progressiverobot/hmailserver/' --certificate-oidc-issuer https://token.actions.githubusercontent.com hMailServer-6.3.3-x64.exe.

From 6.3.1, the Windows installer is also Authenticode-signed by Progressive Robot Ltd, and its release notes give two SHA-256 values, as built and as published. Check against the published value, which is what the cosign bundle covers. SmartScreen still warns on the signed installer. Releases before 6.3.1 and all Linux packages carry no Authenticode signature. A few older files have no Sigstore bundle, including every 6.2.17 and 6.2.18 file and the 6.3.1 x86-64 .deb, so check those by SHA-256 alone. See the downloads page and the documentation.

hMailServer has run on 64-bit Linux, on x86-64 and AArch64, since hMailServer 6.3.0, released on 10 September 2026. The current release, hMailServer 6.3.3 of 15 September 2026, ships a .deb for Debian and Ubuntu and an .rpm for Fedora, RHEL and openSUSE, each installing a systemd service configured under /etc/hmailserver. Arch uses the PKGBUILD in the source tree. The database is PostgreSQL or MySQL/MariaDB. The AppImage is for trying hMailServer out on a laptop, not for running mail.

Linux has no Control Panel, COM API, event scripts or Microsoft SQL Server support, so a Linux server is administered through its configuration file, the hmailserver command line, the REST API and the browser Control Deck. As of hMailServer 6.3.3, the documentation states that per-domain DKIM signing cannot yet be configured on Linux, so a domain that must DKIM-sign its outbound mail belongs on Windows. Moving an existing Windows installation to Linux is not a supported migration. See Running hMailServer on Linux in the documentation and the Linux category.

hMailServer 6.3.3, released on 15 September 2026, supports Windows 10 version 1607 or later, Windows 11, and Windows Server 2016 or newer, on 64-bit (x64) systems only. The Windows download is a single installer, hMailServer-6.3.3-x64.exe, and there has been no 32-bit build since hMailServer 6.0. The documentation asks for at least 2 GB of RAM and 20 GB of free disk space.

Run the installer as an administrator. It upgrades an existing hMailServer 5.3 to 5.7, 6.0, 6.1 or 6.2 installation in place. The Control Panel and the database tools are .NET 10 applications, and the installer adds the .NET 10 Desktop Runtime if it is missing. Windows is the more complete platform: the Control Panel, the COM API and event scripts are available only on Windows, and per-domain DKIM cannot yet be configured on Linux. See chapter 3, "Before you install: the checklist", in the hMailServer documentation, and Installation, migration & upgrades.

hMailServer supports four databases on Windows: the built-in SQL Server Compact database (the default), MySQL or MariaDB, Microsoft SQL Server (including the Express edition) and PostgreSQL. On Linux, supported since hMailServer 6.3.0 (10 September 2026) and in the current release, 6.3.3 (15 September 2026), it supports two: PostgreSQL and MySQL/MariaDB.

The Linux build refuses Microsoft SQL Server and SQL Server Compact by name, with an explanation in the error log, because both are reached through Windows-only interfaces. It was tested end to end against PostgreSQL 18 and MariaDB 11.8. PostgreSQL is reached through libpq, which the package depends on. The MySQL/MariaDB client is loaded at run time and must be installed separately: libmariadb3 on Debian and Ubuntu, mariadb-connector-c on Fedora and RHEL, mariadb-libs on Arch.

The documentation calls the built-in database adequate on Windows for up to a few dozen mailboxes, and lists MySQL/MariaDB for most production use and PostgreSQL for larger installations. See choosing a database and creating the database on Linux.

Moving an hMailServer installation from Windows to Linux is not a supported migration in hMailServer 6.3.3, released 15 September 2026. Both platforms use database schema 6040 and the same message store format, but there is no tested path, so the documentation says to treat the move as a new installation plus a mailbox-level migration, not as a restore.

Known breaks:

• A configuration file with Type=MSSQL or Type=MSSQLCE is refused on Linux; migrate that database to PostgreSQL or MySQL on Windows first.
• DPAPI-protected secrets, including route, fetch-account and relay passwords, private-key passphrases and the administrator's TOTP secret, cannot be read on Linux and must be re-entered.
• Event scripts and COM integrations do not work on Linux.
• Per-domain DKIM is read-only.
• Directory names derived from addresses are lower-cased, and the message store must be owned by hmailserver.

If you try anyway, keep the Windows server running until the copy is verified. See section 35.14 of the hMailServer documentation, or ask in Installation, migration & upgrades or Linux.

hMailServer has a built-in webmail at /portal, served by its REST API listener and compiled into the server binary on both Windows and Linux. A browser mailbox page arrived in hMailServer 6.2.28, it became a full webmail in 6.3.0, and hMailServer 6.3.3, the current release (15 September 2026), rebuilt it in the shape of Gmail and Outlook.com, with inbox tabs, mute, pop-out windows and search history.

It is off by default. Setting RestApiPort in hMailServer.ini turns on the REST API, the Control Deck and the webmail together. The listener will not start while the administrator password is empty, binds to 127.0.0.1 by default, and needs a TLS certificate and key before it can bind to a non-loopback address. Users sign in with their own mailbox address and password; the administrator password and API keys are refused. IMAP clients, or Roundcube pointed at the IMAP service, remain a reasonable alternative. See chapter 36, the version history and Webmail, portal & Control Deck.

hMailServer supports Let's Encrypt through a built-in ACME v2 client that obtains a certificate, assigns it to TLS ports that have none of their own and renews it before expiry, with no scheduled task or manual restart. Use hMailServer 6.2.25 or later on Windows, or 6.3.0 or later on Linux; the current release is 6.3.3. Releases up to 6.2.21 never obtained a certificate from Let's Encrypt, and 6.2.24 stopped the service whenever a certificate was issued or renewed.

The client is off by default. Set AcmeEnabled=1, AcmeContactEmail and AcmeDomains in the Control Panel under Settings, Security, Certificates (ACME), or in Bin\hMailServer.INI. Linux has no Control Panel, so use /etc/hmailserver/hMailServer.ini.

Validation uses HTTP-01, so port 80 must be reachable from the internet and every name in AcmeDomains must already resolve to the server. Keep AcmeReuseKey=1 (the default) so published DANE TLSA records stay valid across renewals. See the hMailServer documentation (sections 9.1 and 35.9) and TLS, certificates & deliverability.

hMailServer supports SPF, DKIM, DMARC, MTA-STS and DANE, plus ARC sealing and TLS-RPT. MTA-STS, DANE, DMARC, ARC, TLS-RPT and Ed25519 DKIM arrived in hMailServer 6.0.0.

For outbound mail, MtaStsEnabled, DaneEnforcementEnabled and DnssecValidationEnabled are on by default. If a recipient domain has published a policy, it must present verified TLS or the message is deferred rather than sent unencrypted. To host your own MTA-STS policy, turn on the web-services listener with WebServicesHttpsPort=443.

hMailServer signs outgoing mail with RSA or Ed25519 DKIM keys, and scores inbound SPF, DKIM and DMARC failures as spam tests. You still publish your own SPF, DKIM and DMARC records in DNS.

Caveats: ARC sealing is off by default and seals only mail from hosted, DKIM-enabled domains. TLS-RPT reports are sent only once TlsRptFromAddress is set. Per-domain DKIM signing cannot yet be configured on Linux, so a domain that must DKIM-sign its outbound mail should stay on Windows for now.

See the hMailServer documentation (chapters 8 and 9) and TLS, certificates & deliverability.

hMailServer has had a REST API since version 6.0.0, served under /api/v1/ and described by an OpenAPI 3.0.3 document at GET /api/v1/openapi.json. It gained a large write surface in 6.3.0 (10 September 2026). In the current release, 6.3.3 (15 September 2026), the project's parity check counts 328 of the 330 properties the Windows Control Panel writes as writable over REST; only account groups and their members are not.

The API is off by default on Windows and Linux. Setting RestApiPort in the [Settings] section of hMailServer.ini turns it on, bound to 127.0.0.1 by default. It will not start with an empty administrator password, and on any address other than 127.0.0.1, localhost or ::1 it needs a TLS certificate and key. Credentials are the administrator password, scoped API keys, session cookies and a mailbox's own login, which reaches only /api/v1/me and sign-in. Linux has no Control Panel or COM, so there the REST API and the Control Deck it serves at / do that job. See the REST API chapter and REST API & integrations.

hMailServer is not a replacement for Microsoft Exchange: it is a mail server for SMTP, IMAP and POP3, with no shared calendaring, no Teams integration and no ActiveSync. The current release, hMailServer 6.3.3 (15 September 2026), is free and open source under the AGPL-3.0 licence, with no per-mailbox fees, and runs on Windows and Linux.

It hosts mail for many domains, with spam filtering, virus scanning through ClamAV, a rules engine and Sieve filtering. Since 6.3.3 it also serves each account's address book over CardDAV once the web services HTTPS listener is turned on.

Users read mail in Outlook, Thunderbird, Apple Mail, on a phone or in the built-in webmail at /portal once the REST listener is turned on. The documentation suggests it for small businesses, developers needing a real SMTP and IMAP test endpoint and hosting providers. If nobody on staff is comfortable editing DNS records and watching a log file, it says a hosted mailbox provider will serve them better. See chapter 1 of the hMailServer documentation.

Free help with hMailServer is available on the hMailServer community forum, run by Progressive Robot Ltd, which forked hMailServer in 2026 and maintains the hMailServer 6 line. hMailServer is free under AGPL-3.0, and no answer on the forum is gated behind a support plan.

Post general problems in General support; Linux, TLS, certificates & deliverability and REST API & integrations have their own categories. Follow the how to ask template: the full version with its build stamp, the platform, the database, how the install got there, and a log excerpt with timestamps intact and credentials redacted.

The hMailServer documentation runs to 37 chapters. Reproducible defects go on the GitLab issue tracker, linked from the forum thread. Never post security vulnerabilities publicly: email [email protected] with the version, the platform and enough detail to reproduce the problem.

For paid help, Progressive Robot offers deployment, migration, hardening and fully managed hMailServer on Windows and Linux. See support plans, Managed hMailServer or contact Progressive Robot.

Report a security vulnerability in hMailServer privately by email to [email protected], not in a forum thread or a public GitLab issue. Include the hMailServer version, the platform and enough detail to reproduce the problem.

Progressive Robot will acknowledge the report, tell you which release carries the fix, and credit you in the release notes unless you would rather not be named. Security advisories are posted in Announcements & releases. This process is rule 6 of the forum rules.

The hMailServer 6 source code is on GitLab at gitlab.com/Progressiverobot/hmailserver, under the AGPL-3.0 licence. Progressive Robot Ltd forked hMailServer in 2026 and maintains the hMailServer 6 line in that repository. The original project, created by Martin Knafve, is separate and has resumed development.

One source tree builds the 64-bit Windows server with Visual Studio 2026 and MSVC. Since hMailServer 6.3.0 it also builds the Linux server with clang or GCC. Every release from 6.0.0 to the current 6.3.3 has a tag. Since 6.2.24, two clean builds of each release have produced a byte-identical hMailServer.exe, so anyone with the same toolchain can rebuild a tag and compare the result. The Linux build steps are in supported platforms and packages. Reproducible defects go on the GitLab issue tracker.

No. The hMailServer community forum at www.hmailserver.co.uk is separate from the forum at hmailserver.com, which belongs to the original hMailServer project created by Martin Knafve. This forum is run by Progressive Robot Ltd, which forked hMailServer in 2026 and maintains the hMailServer 6 line. The current release is hMailServer 6.3.3, released on 15 September 2026 for Windows and Linux under the AGPL-3.0 licence.

The original project went quiet for years and has since resumed development separately. Accounts and posts from hmailserver.com were not carried over, so register here to post. Staff post under their own accounts, chris and Progressiverobot.

Release notes, security advisories and forum notices are posted in Announcements & releases. Support questions go in categories such as General support, Installation, migration & upgrades, Linux and TLS, certificates & deliverability. Before posting, read the forum rules and the template on posting a support request.