RFC1891 specifies that the keyword ENVID can be given to the MAIL FROM: command:

MAIL From:<address> ENVID=envelopeID

ENVID is used to propagate a consistent envelope identifier (distinct from the Message-ID: header; see Message-ID:) that will be permanently associated with the message. The envelopeID can contain any ASCII characters between ! and ~, except + and =. Any characters outside that range must be encoded by prefixing an uppercase, two-digit, hexadecimal representation of it with a plus. For example, an envelopeID composed of the letter X followed by a delete character would be encoded like this:

X+7F

When mail is received over an SMTP channel and an ENVID identifier is specified, that identifier is saved as part of the envelope information. The value of the ENVID identifier is saved in and restored from the qf file's Z line (Z line). For bounced mail, the ENVID identifier is printed with the Original-Envelope-Id: DSN header (see RFC1894) as part of the DSN MIME body. Beginning with V8.8 sendmail, an ENVID identifier can also be assigned to a message with the -V command-line switch (-V).

The ${envid} macro is set only during delivery. By contrast, the ${dsn_envid} macro (${dsn_envid}) is set when mail is received via SMTP and when the -V command-line switch (-V) is used to set the envelope identifier.

When mail is delivered, the value of the envelope's ENVID identifier is saved in the ${envid} macro. That macro is available for use with delivery agents that understand DSN.

${envid} is transient. If it is defined in the configuration file or in the command line, that definition can be ignored by sendmail. Note that a $& prefix is necessary when you reference this macro in rules (that is, use $&{envid}, not ${envid}).