The total volume of messages currently stored in the MTA,
measured in kilo-octets. This volume should include all
stored data that is logically above the mail transport
protocol level. For example, an SMTP-based MTA should
use the number of kilo-octets in the message header and
body, while an X.400-based MTA would use the number of
kilo-octets of P2 data. This includes messages that are
awaiting transmission to some other MTA or are waiting
for delivery to an end-user or application.