Dovecot
Dovecot is an excellent choice for both small and large installations.
Dovecot is among the best performing IMAP servers while still supporting the standard mbox and Maildir formats. The mailboxes are transparently indexed, which gives Dovecot its good performance while still providing full compatibility with existing mailbox handling tools.
Dovecot is standards compliant. Dovecot passes all IMAP server standard compliancy tests while most other servers fail many of them.
Dovecot’s indexes are self-optimizing. They contain exactly what the user’s client commonly needs, no more and no less.
Dovecot is self-healing. It tries to fix most of the problems it notices by itself, such as broken index files. The problems are however logged so the administrator can later try to figure out what caused them.
Dovecot tries to be admin-friendly. Common error messages are made as easily understandable as possible.
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Aid4Mail
Aid4Mail is a fast, reliable, and highly accurate tool to collect, recover, search, and convert emails. It supports most mailbox file formats (e.g. PST, OST, OLM, mbox), IMAP accounts (e.g. Yahoo! Mail, AOL) and popular mail service providers (e.g. Microsoft 365, Exchange, Gmail). Aid4Mail can recover double-deleted messages and corrupted emails, and extract MIME data from certain types of unknown file formats through file carving.
Aid4Mail provides a large array of tools to search and filter out unwanted emails during conversion. Save time by using native pre-acquisition filters to download a subset of your mail from Exchange, Office 365, Gmail and other webmail services. Use Aid4Mail’s integrated search engine to cull-down your email collection. Its search operators are very similar to Gmail and Office 365.
Aid4Mail is used by Fortune 500 companies, government agencies and legal professionals around the world. It is made in Switzerland by perfectionists.
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Hermes SEG
Hermes SEG is a free, open-source secure email gateway and server. It delivers spam, virus, and malware protection using SpamAssassin, ClamAV, and Amavisd-new, along with full encryption in transit and at rest via SMTP TLS, S/MIME, PGP, encrypted PDF (CipherMail), and Dovecot mail-crypt. It includes email archiving and integrated mailbox hosting with quotas, aliases, shared folders, Sieve rules, vacation replies, and mobile auto-configuration. Collaboration features come through Nextcloud (webmail, file sync, calendars, contacts). Identity and access are managed with OpenLDAP and Authelia, supporting SSO and MFA (TOTP, WebAuthn, Duo). It also enforces modern authentication standards: SPF, DKIM, DMARC, and ARC.
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Haraka
Haraka can serve thousands of concurrent connections and deliver thousands of messages per second. Haraka and plugins are written in asynchronous JS and are very fast. Haraka has very good spam protection (see plugins) and works well as a filtering MTA. It also works well as an MSA running on port 587 with auth and dkim_sign plugins enabled. Haraka makes no attempt to be a mail store (like an exchange or postfix/exim/qmail), an LDA, or an IMAP server (like Dovecot or courier). Haraka is typically used with such systems. Haraka has a scalable outbound mail delivery engine built in. Mail marked as relaying (such as via an auth plugin) is automatically queued for outbound delivery. Haraka's plugin architecture provides an easily extensible MTA that complements traditional MTAs that excel at managing mail stores but do not have sufficient filtering.
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