Courier
The Courier mail transfer agent (MTA) is an integrated mail/groupware server based on open commodity protocols, such as ESMTP, IMAP, POP3, LDAP, SSL, and HTTP. Courier provides ESMTP, IMAP, POP3, webmail, and mailing list services within a single, consistent, framework. Individual components can be enabled or disabled at will. The Courier mail server now implements basic web-based calendaring and scheduling services integrated in the webmail module. Advanced groupware calendaring services will follow soon.
The Courier mail server's source code should compile on most POSIX-based operating systems based on Linux, and BSD-derived kernels. The Courier mail server should also compile on Solaris and AIX, with some help from Sun's or IBM's freeware add-on tools for their respective operating systems.
The Courier mail server evolved out of several related projects, that merged together (more on that later). The Courier mail server implements SMTP extensions for mailing list management.
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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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Zentyal
Join Windows® clients to the domain and manage them transparently. No disruption to your users, no user or device CALs. Comes with industry-standard SMTP and POP3/IMAP mail server built upon the most established technologies and protocols. Guarantees reliable and secure Internet access and helps to manage all the basic network infrastructure services. Receive software updates until the EOL-date of each version and get support when needed. Zentyal offers an easy to use Windows Server® alternative. It comes with native compatibility with Microsoft Active Directory® allowing you to join Windows® clients to the domain and manage them easily, causing no disruption to your users. Zentyal includes the industry-standard SMTP and POP3/IMAP mail servers built upon the most established technologies and protocols. Gives you the opportunity to deploy Zentyal as a mail server, domain & directory server with mail or all-in one server.
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hMailServer
hMailServer is a free, open source, e-mail server for Microsoft Windows. It's used by Internet service providers, companies, governments, schools and enthusiasts in all parts of the world.
It supports the common e-mail protocols (IMAP, SMTP and POP3) and can easily be integrated with many existing web mail systems. It has flexible score-based spam protection and can attach to your virus scanner to scan all incoming and outgoing email.
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