sendmailcf

sendmail.cf is the runtime configuration file used by the Sendmail Mail Transfer Agent (MTA). It governs mail processing, routing, and delivery decisions and encodes policies for domains, hosts, and users. The file is read by the sendmail daemon when it starts or reloads, and errors in its syntax can prevent mail from being delivered.

In most modern deployments, sendmail.cf is not edited directly. Administrators modify a higher-level file named sendmail.mc (the m4 macro-based source) and then generate sendmail.cf by processing sendmail.mc with the M4 macro processor. This workflow supports consistent configuration management and easier reuse. Some older installations may still maintain sendmail.cf directly, but that approach is discouraged because it is harder to maintain and prone to error.

Location and structure: sendmail.cf is typically found at /etc/mail/sendmail.cf or /etc/sendmail.cf, depending on the operating system. The file consists of macro definitions, rulesets, and class declarations that control how addresses are rewritten, routed, and delivered. It defines the available mailers (for example SMTP) and can embed access controls, routing instructions, masquerade parameters, and mappings from domain or user to delivery behavior. The syntax is specialized and compact, and changes can have broad effects on mail flow.

Maintenance and usage: applying changes requires reloading or restarting the Sendmail service so that the new configuration is read. Because of its central role in mail delivery, configuration changes are typically tested in a staging environment and validated before deployment.