The Postfix system itself also reports problems to the postmaster alias. You may not be interested in all types of trouble reports, so this reporting mechanism is configurable. The default is to report only serious problems (resource, software) to postmaster:

Default: resource, software

The meaning of the classes is as follows:

• bounce: Send postmaster copies of undeliverable mail.
• 2bounce: Send double bounces to the postmaster.
• delay: Inform the postmaster of delayed mail. In this case, the postmaster receives message headers only.
• policy: Inform the postmaster of client requests that were rejected because of (UCE) policy restrictions. The postmaster receives a transcript of the entire SMTP session.
• protocol: Inform the postmaster of protocol errors (client or server side) or attempts by a client to execute unimplemented commands. The postmaster receives a transcript of the entire SMTP session.
• resource: Inform the postmaster of mail not delivered due to resource problems (for example, queue file write errors).
• software: Inform the postmaster of mail not delivered due to software problems.