Release 5677: SMTPD limits on low disk space

In this release we have tuned a couple of postfix configurations to safeguard Hypernodes from excessive e-mail volumes. What may affect users is that when the root disk has free space less than 15MB, SMTPD would not accept new requests. We have also adjusted the active queue size in proportion to the enforced mail limits, to avoid unnecessary operations and reduce resource consumption of postfix queue manager process.