The MAIL ESMTP command can
optionally be followed by an RFC1891 ENVID envelope identifier:
MAIL From: <address> ENVID=envelopeID
ENVID is used to propagate a consistent envelope identifier (distinct
from the Message-ID: header, Message-ID:) that is permanently associated with a
message.
The Z line holds that ENVID envelope identifier
information:
ZenvelopeID
The ENVID information needs to be held separately from the
S sender line because
sendmail has no way to determine in advance
whether a recipient host speaks ESMTP.
There must be only a single Z line in any
qf file. The ${envid}
sendmail macro (${envid})
also stores the ENVID value.
