have callout sender verification applied.
* **grey_users**: a simple list of local parts that should have
greylisting applied.
+* **localonly**: a simple list of local parts that should only accept mail
+ from debian.org hosts.
* **neversenders**: a simple list of local parts (of the virtual domain) that
should never appear as senders of mails.
* **whitelist**: a colon separated list starting with "localpart:" of hosts