Ordinarily, masquerading (Section 4.4) affects only the headers of email messages, but sometimes it is also desirable to masquerade the envelope.^[8] For example, error messages are often returned to the envelope-sender address. When many hosts are masquerading as a single host, it is often desirable to have all error messages delivered to that central masquerade host.

^[8] See Section 1.5.4 for a description of the envelope and how it differs from headers.

The masquerade_envelope feature causes masquerading to include envelope addresses:

MASQUERADE_AS(`our.domain')     masquerade headers
FEATURE(`masquerade_envelope')  also masquerade the envelope

These mc lines cause all envelope addresses (where the host part is declared as part of class $=w; $=w) to be transformed into our.domain. See MASQUERADE_DOMAIN for a way to also masquerade other domains, and see the masquerade_entire_domain feature for a way to also masquerade all the hosts under other domains.

In general, masquerade_envelope is recommended for uniform or small sites. Large or variegated sites might prefer to tailor the envelope on a subdomain-by-subdomain or host-by-host basis.