OpenSMTPD

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.

Completely rewritten relay restrictions with an easier to use interface that gives finer control over relay security. New domain name resolver (DNR), EIMS now has it's own DNR, which works around bugs in the Open Transport DNR and performance problems with the OS X DNR. The new DNR does not currently support non-recursive DNSs. SMTP Submit port, this is a second SMTP port running on TCP port 587 by default. It currently works exactly the same as the standard SMTP port. This is handy for clients that are stuck behind an ISP that blocks port 25 access. Sending Setup option for maximum recipients per message. Expand SMTP Addresses option, you can now turn this off, with EIMS 3.1 and earlier it was always on. The SMTP server (both port 25 and 587) now reports Telnet connections and connections that look like port scans (connections where no data is sent and it doesn't time out).