As part of the RFC2554 authentication scheme, a client can ask if a user other than the envelope sender is allowed to authenticate on behalf of the envelope sender. This is done by adding an AUTH= parameter to the MAIL FROM: keyword:

MAIL FROM: <user@host.domain> AUTH=address

This ${auth_author} macro is assigned the address that followed the MAIL FROM: AUTH= extension.

The ${auth_author} macro is useful for adding your own rules to the Local_trust_auth rule set. Note that a $& prefix is necessary when you reference this macro in rules (that is, use $&{auth_author}, not ${auth_author}).

${auth_author} is transient. If defined in the configuration file or in the command line, that definition can be ignored by sendmail.