The Sender: header is like the From: header. But whereas the From: header shows the address of one sender (RFC822) or many authors (RFC2822), the Sender: header shows the address of the actual sender. For example, an assistant can mail a letter for the boss using the boss's account. The boss's address is in the From: header, and the assistant's address is in the Sender: header. See the From: header (From:) for a description of the syntax and rules for this Sender: header.

Newer MUAs allow the user to create a custom Sender: header. The Sender: header should never be declared in the configuration file.