Exim 4.5 for Mac could be downloaded from the developer's website when we last checked. We cannot confirm if there is a free download of this app available. The program relates to System Tools. Please check the Mac app with an antivirus before launch as it is downloaded from the developer's website, and we cannot ensure that it is safe.
From the developer:
Exim 4.5 is a powerful MTA (mail transport agent) for UNIX machines, which includes Mac OS X. This is useful as a personal SMTP server, or as a high-load mail server. It's ACL allows flexibles configurations, and advanced capabilities, as mail content filter, antispam and antivirus integration, flexible redirection, support for local UNIX mail, user autethication, etc... This package helps installing and using Exim's port for Darwin/OSX.