You can stop and start mail service on the hosting appliance.
To have all of your outgoing mail sent through a central relay
server, specify a Smart relay hostname.
Use the Reject... and RBL options to strengthen security for your
mail service. Separate multiple domain names with a semi-colon (;).
Enter maximum file sizes for incoming and outgoing messages to
limit bandwidth usage for all domains hosted on the appliance.
(Between 0 and 999 MB. Note that entering 0 will disable the mail
service.)