Mox is a modern full-featured open source secure mail server for low-maintenance self-hosted email. Features:

Quick and easy to start/maintain mail server, for your own domain(s). SMTP (with extensions) for receiving, submitting and delivering email. IMAP4 (with extensions) for giving email clients access to email. Webmail for reading/sending email from the browser. SPF/DKIM/DMARC for authenticating messages/delivery, also DMARC aggregate reports. Reputation tracking, learning (per user) host-, domain- and sender address-based reputation from (Non-)Junk email classification. Bayesian spam filtering that learns (per user) from (Non-)Junk email. Slowing down senders with no/low reputation or questionable email content (similar to greylisting). Rejected emails are stored in a mailbox called Rejects for a short period, helping with misclassified legitimate synchronous signup/login/transactional emails. Internationalized email, with unicode in email address usernames ("localparts"), and in domain names (IDNA). Automatic TLS with ACME, for use with Let's Encrypt and other CA's. DANE and MTA-STS for inbound and outbound delivery over SMTP with STARTTLS, including REQUIRETLS and with incoming/outgoing TLSRPT reporting. Web admin interface that helps you set up your domains, accounts and list aliases (instructions to create DNS records, configure SPF/DKIM/DMARC/TLSRPT/MTA-STS), for status information, and modifying the configuration file. Account autodiscovery (with SRV records, Microsoft-style, Thunderbird-style, and Apple device management profiles) for easy account setup (though client support is limited). Webserver with serving static files and forwarding requests (reverse proxy), so port 443 can also be used to serve websites. Simple HTTP/JSON API for sending transaction email and receiving delivery events and incoming messages (webapi and webhooks). Prometheus metrics and structured logging for operational insight. "mox localserve" subcommand for running mox locally for email-related testing/developing, including pedantic mode. Most non-server Go packages mox consists of are written to be reusable.

Mox is available under the MIT-license and was created by Mechiel Lukkien, mechiel@ueber.net. Mox includes BSD-3-claused code from the Go Authors, and the Public Suffix List by Mozilla under Mozilla Public License, v2.0. Mox has automated tests, including for interoperability with Postfix for SMTP. Mox is manually tested with email clients: Mozilla Thunderbird, mutt, iOS Mail, macOS Mail, Android Mail, Microsoft Outlook. Mox is also manually tested to interoperate with popular cloud providers: gmail.com, outlook.com, yahoo.com, proton.me. The code is heavily cross-referenced with the RFCs for readability/maintainability.