Sendmail

It is a self‑hosted mail transfer agent that routes email between servers and delivers messages to local mailboxes. The software implements standard SMTP protocols and supports common extensions such as DKIM for message signing. It can be deployed on‑premises without a subscription and is released under the Sendmail license.

Administrators of large or complex email infrastructures use it to manage inbound and outbound mail flow, integrate with existing directory services, and enforce routing policies. The open‑source version provides the core MTA functionality needed for enterprise‑scale deployments.

What distinguishes this implementation is its alignment with Proofpoint’s broader security ecosystem, offering optional AI‑driven threat protection and integration points for additional security services while remaining a stable, free‑tier solution for organizations that prefer to control their own email infrastructure.