sendmailmc

Sendmailmc is the macro configuration file used with the Sendmail Mail Transfer Agent (MTA). In practice, it is typically named sendmail.mc and is processed by the m4 macro processor to generate the runtime Sendmail configuration file, sendmail.cf. The mc file acts as the editable source for the MTA’s behavior, routing, and policy decisions, while the resulting cf file is what the daemon actually loads at startup or on reload. The file is usually located under /etc/mail or a distribution-specific path, depending on the operating system.

Administrators edit sendmail.mc to enable features, specify domain handling, set routing rules, and control security options. Because it is processed by m4, the syntax is macro-based and requires careful formatting. After making changes, the standard workflow is to run m4 on the mc file to produce sendmail.cf, and then reload or restart the Sendmail service. Some Linux distributions provide wrapper tools (such as update-mta or a Makefile target) to automate this build step.

Contents and common usage: A typical sendmail.mc includes macro directives and features such as masquerading (to rewrite outgoing domains), domain and host definitions, access controls, and paths to auxiliary files (like virtusertable or auth info). Examples include directives that set the local domain, enable or disable particular features, and reference additional configuration files. The exact lines vary by site and security requirements, but the mc file is generally the single source of truth for configuration, with sendmail.cf reflecting its processed output.

Context: sendmail.mc represents a configuration approach introduced to simplify management of Sendmail’s complex cf syntax. It remains widely used in many Unix-like systems, though newer MTAs offer alternative configuration methods.