Dovecot

Dovecot implements IMAP and POP3 services with a focus on security, offering a fully open‑source, self‑hostable backend that adheres to relevant RFCs. It supports the traditional mbox and Maildir mailbox formats and transparently indexes messages to provide high performance while remaining standards‑compliant, passing all IMAP server conformance tests.

The server’s authentication subsystem is highly flexible, allowing integration with various authentication databases and mechanisms, and can be used directly by MTAs such as Postfix, Exim, or OpenSMTPD for SMTP authentication. Indexes are self‑optimizing, storing only the data typically required by client applications, which helps reduce resource usage and simplify scaling.

Targeted at administrators of email infrastructure—including ISPs, telcos, and hosting providers—Dovecot provides a stable, MIT‑licensed platform that can be deployed without subscription fees and is supported through a public mailing list and extensive documentation.