GoESMTP is a multi-node ESMTP server. It was designed from the start to work as a cluster. Each node keeps an index of the available messages by using hashes which are distributed throughout the cluster.
OpenProtect is an anti-spam server that filters spam and blocks viruses, phishing, and other malware. Acts as an SMTP gateway and integrates with any email server like Postfix, Sendmail, qmail, Exim, Microsoft Exchange, Lotus Domino, Zimbra, etc.