MS Exchange, Qmail and Sendmail

Microsoft Exchange Server

Microsoft Exchange Server is an email management server widely used by companies that also employ Microsoft's Enterprise Solutions. The software supports electronic mail, calendar synchronization and management, contact management and task management. It supports mobile access to this information, as well as web-based access.

Qmail

Qmail is an MTA that runs on the UNIX operating system. It was written by Dan J. Bernstein as a replacement for Sendmail, touting more security, and the added benefit of being a license-free product.

Sendmail

Sendmail is an MTA that is open-source and license-free and that runs on UNIX. Its drawbacks, and reason for eventual replacement in many areas by Qmail, lie in security issues and vulnerabilities. Sendmail is a direct "descendant" of an original mail delivery system utilized on the ARPANET in its early days.