With over 7,100 GitHub stars and 200 contributors since 2016, Mailu has become one of the most popular self-hosted email solutions, delivering a complete production-ready mail server stack as pre-configured Docker containers that eliminate weeks of manual setup. The platform bundles Postfix for SMTP and Submission with auto-configuration profiles for Thunderbird, Outlook, and mobile clients, Dovecot for IMAP and POP3 with full-text search across email attachments, Rspamd for intelligent spam filtering with auto-learn Bayesian classification, greylisting, SPF and DMARC validation, and ClamAV for antivirus scanning of attachments with configurable malicious file blocking. The web administration interface provides global admin controls, per-domain delegation, user quota management, domain aliases, custom routing rules, and announcement capabilities. Users access self-service features including aliases, auto-reply, auto-forward, fetched accounts from external mailboxes via fetchmail, and Sieve filter management through ManageSieve. Security features include enforced TLS on all connections, automatic Let's Encrypt certificate provisioning, outgoing DKIM signing, DANE and MTA-STS support for transport security, and Snuffleupagus PHP hardening for the web interface. The setup wizard at setup.mailu.io generates complete Docker Compose configurations tailored to specific deployment requirements. All components are free and open-source software with zero proprietary dependencies and no tracking. Running on a dedicated VPS on RepoCloud with guaranteed CPU, RAM, and SSD, full root SSH access, and a browser serial console. MIT licensed.

Calendars, to-do lists, journal entries, and contacts, synced over the open CalDAV and CardDAV standards nearly every client already speaks: Radicale is a small pure-Python server that works with Thunderbird, DAVx5 on Android, Apple Calendar and Contacts, GNOME, and many more. Its defining design choice is radical simplicity: there is no database. Events live as plain .ics files and contacts as .vcf files in an ordinary folder structure, which makes backup a copy command, migration a move, and disaster recovery a matter of reading text files. The server works out of the box with no complicated setup, then grows as needed: flexible authentication (htpasswd files among other methods), per-collection authorization rules, TLS-secured connections, and a plugin system for extending storage, auth, and rights handling. Built-in limits on parallel connections, file sizes, and failed authentication attempts harden it for network exposure behind a reverse proxy. A bundled web interface handles creating and managing calendars and address books - useful since many clients cannot create collections themselves. Maintained since 2011 with 140+ contributors, GPLv3-licensed, and light enough to run on the smallest VPS or a Raspberry Pi.

Kurrier unifies email, calendars, contacts, and file storage in a single web interface that connects to any email provider you already use. The Next.js frontend with Nitro API worker deploys via Docker Compose on PostgreSQL and Redis, delivering a cohesive workspace where IMAP/SMTP mailboxes from Gmail, Outlook, or corporate servers sit alongside transactional providers like AWS SES, SendGrid, Mailgun, and Postmark. Automatic inbox building handles threads, attachments, sent items, and deletion sync through persistent background polling. The CalDAV backend enables full two-way calendar synchronization with iOS, macOS, Android via DAVx5, and Thunderbird, supporting event invitations, RSVP processing through the inbox, timezone-aware scheduling, and contact-aware attendee linking. CardDAV contacts sync bidirectionally across all devices with unified name, avatar, and email resolution. Kurrier Drive provides integrated file storage through WebDAV locally or S3-compatible backends for cloud storage. Multiple identities and domains let you manage several email accounts from different providers in one interface, with fast local search indexing subjects, senders, and content without dependency on external APIs. The API-first architecture exposes clean endpoints for automation and custom integrations. Running on a dedicated VPS on RepoCloud with guaranteed CPU, RAM, and SSD, full root SSH access, and a browser serial console. AGPL-3.0 licensed.

Two decades of continuous development made Roundcube the standard-bearer of open-source webmail - a PHP IMAP client that gives any mail server a polished, application-like interface in the browser. It connects to whatever IMAP/SMTP stack you run - Dovecot, Postfix, a hosted mailbox - and delivers the full desktop-client experience: drag-and-drop message management, threaded conversation views, full MIME and HTML mail handling, find-as-you-type address book with groups and LDAP connectors, multiple sender identities, full-text search, and spell checking in dozens of languages. The default Elastic skin is genuinely responsive, working as well on a phone as a desktop, and the entire UI is skinnable. The plugin API is where deployments get shaped: managesieve exposes server-side filter management in the UI, enigma brings PGP encryption and signing via GnuPG, markasjunk trains spam filters, zipdownload batches attachments, password lets users change credentials, and attachment_reminder catches the classic forgotten-attachment email - among hundreds of community plugins. Built-in caching keeps large mailboxes fast, IMAP ACLs and shared folders support team setups, and XSS protection is engineered into the rendering pipeline. It scales from a single personal mailbox to unlimited users, backed by MySQL, MariaDB, PostgreSQL, or SQLite. GPL-licensed with regular security releases.

Stalwart replaces the traditional Postfix + Dovecot + SpamAssassin + calendar-server stack with one Rust binary that speaks every standard mail and collaboration protocol natively. JMAP, IMAP4rev2, POP3, SMTP, CalDAV, CardDAV, and WebDAV all run inside the same process — no glue scripts, no sidecar daemons, no version conflicts between components. The pluggable storage architecture lets operators choose RocksDB for single-node deployments, FoundationDB for distributed clusters, PostgreSQL, MySQL/MariaDB, or SQLite for the data store, S3/MinIO/Azure Blob for message blobs, and Elasticsearch or Meilisearch for full-text search, with Redis or the internal engine backing rate limiters and session state. Security features include S/MIME and OpenPGP encryption at rest, automated DKIM key generation with DNS publication, DANE and MTA-STS transport security, automatic ACME TLS provisioning, granular ACLs, rate limiting, and IP banning. The browser-based admin console manages accounts, domains, groups, mailing lists, SMTP queues, DMARC/TLS-RPT/ARF reports, and every configuration object without touching a config file, while the self-service portal at /account gives end users password reset and encryption key management. Multi-tenant support with per-tenant quotas enables hosting-platform deployments, and coordinator-less clustering via Zenoh or NATS scales horizontally by adding nodes. Deploy via Docker or the standalone binary. Running on a dedicated VPS on RepoCloud with guaranteed CPU, RAM, and SSD, full root SSH access, and a browser serial console. AGPL v3 licensed.

With 14,000 GitHub stars and 81 releases since March 2023, Stalwart is the most protocol-complete open-source mail server available — delivering JMAP, IMAP4rev2, IMAP4rev1, POP3, SMTP, CalDAV, CardDAV, and WebDAV from a single Rust binary that compiles to a memory-safe, zero-garbage-collection executable with predictable latency under load. The SMTP server implements DMARC, DKIMv2, DKIMv1, SPF, and ARC for complete message authentication with automatic DKIM key rotation, while transport security enforces DANE, MTA-STS, and SMTP TLS reporting to prevent downgrade attacks. Built-in spam filtering with statistical classifiers, DNS blocklists, and collaborative reputation databases eliminates the need for external Rspamd or SpamAssassin deployments. Encryption at rest protects stored messages with S/MIME or OpenPGP, and automatic TLS certificate provisioning via ACME supports TLS-ALPN-01, DNS-01, and HTTP-01 challenges without manual certificate management. The ManageSieve server enables server-side email filtering rules, while full-text search indexes message bodies and attachments for instant retrieval. Pluggable storage backends support RocksDB for embedded deployments, PostgreSQL, MySQL, and S3-compatible object storage for distributed architectures. LDAP and SQL-based authentication integrate with existing directory services, and the web administration panel manages domains, accounts, quotas, and DKIM keys. Security audited with memory safety guaranteed by Rust's ownership model. Running on a dedicated VPS on RepoCloud with guaranteed CPU, RAM, and SSD, full root SSH access, and a browser serial console. AGPL-3.0 licensed.

Mail-in-a-Box turns a single Ubuntu server into a fully functional email system with one setup command, installing and configuring Postfix for SMTP, Dovecot for IMAP, Roundcube for webmail, and Z-Push for Exchange ActiveSync push notifications in an automated sequence that would take days to replicate manually. The DNS server auto-configures the full suite of email authentication records: SPF, DKIM, DMARC, DNSSEC with DANE TLSA, MTA-STS, and SSHFP. Getting all of these right is what separates email that lands in inboxes from email that lands in spam, and most manual installations never achieve the complete set. Nextcloud integration adds CardDAV contact sync and CalDAV calendar sharing across devices. Let's Encrypt certificates provision and renew automatically for every hosted domain. The web control panel manages users, aliases, custom DNS records, and automated backups, protected by TOTP two-factor authentication. Daily health checks verify that services run correctly, ports remain open, TLS certificates stay valid, and DNS records resolve properly, alerting you before delivery problems develop. Multiple domains and users operate from one installation with internationalized domain name support. Built-in HTTPS static site hosting uses the TLS infrastructure already in place. Runs on a dedicated RepoCloud VPS with guaranteed resources, root SSH, and browser serial console for complete email sovereignty. CC0 public domain licensed.

With over 13,000 GitHub stars and active monthly releases, Mailcow delivers a production-ready mail server infrastructure through Docker Compose orchestration of twelve integrated containers. The stack pairs Postfix as the MTA handling SMTP traffic with Dovecot for IMAP and POP3 retrieval featuring the Flatcurve full-text search engine, while Rspamd provides machine-learning-powered spam filtering with Bayesian classification, DKIM signing, and ARC support. ClamAV scans attachments for malware including Office document macro analysis via Olefy, and SOGo serves as a responsive webmail client with CalDAV, CardDAV, and Microsoft ActiveSync synchronization. The admin panel manages multiple domains, per-user spam score thresholds, quarantine review, black and whitelists, temporary spam aliases, mailbox quotas, and TLS enforcement policies for both inbound and outbound messages. Security features include two-factor authentication via TOTP, WebAuthn, and Yubikey OTP, a Fail2ban-like Netfilter integration for brute-force protection, and automatic SPF, DKIM, and DMARC configuration. The ACME container handles Let's Encrypt certificate generation with DNS-01 challenge support, while Postfix TLS-Pol provides MTA-STS and DANE/TLSA resolution for encrypted transport. Built-in imapsync enables periodic migration from remote mailboxes, and the REST API allows programmatic management of all server resources. MariaDB stores user data, Redis caches configuration and sessions, and Unbound provides local DNSSEC-validating DNS resolution. Running on a dedicated VPS on RepoCloud with guaranteed CPU, RAM, and SSD, full root SSH access, and a browser serial console. GPL-3.0 licensed.

Mox offers a complete mail server stack in a single Go binary requiring no external dependencies. The quickstart command configures a working mail server with SMTP, IMAP4, webmail, and full DNS authentication in under ten minutes. SMTP handling includes a delivery server on port 25, a submission server for authenticated clients, and a queue with automatic retries and DKIM signing. IMAP4rev2 implementation provides full mailbox synchronization with CONDSTORE and QRESYNC for efficient offline clients, NOTIFY for multi-mailbox monitoring, MULTISEARCH across mailboxes, and TLS client certificate authentication via the EXTERNAL SASL mechanism. The built-in webmail provides browser-based reading and composing with message threading, attachments, and HTML rendering without requiring a separate web client. Email authentication implements SPF validation, DKIM signing and verification with automatic key rotation, DMARC policy enforcement with aggregate and failure reporting, DANE with DNSSEC-protected TLSA records, and MTA-STS for certificate verification. Junk filtering combines reputation-based sender scoring with Bayesian content analysis trained per account. The web administration interface manages domains, accounts, DNS records, TLS certificates, delivery queue, and real-time log viewing. Internationalized email addresses with EAI and IDNA support handle non-ASCII domains and mailboxes. Account autoconfiguration publishes settings for Thunderbird autoconfig and Outlook autodiscover. Running on a dedicated VPS on RepoCloud with guaranteed CPU, RAM, and SSD, full root SSH access, and a browser serial console. MIT licensed.