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.