The following policy is now implemented:

    * users can use their SASL login name as sender address;
    * alias and/or list owners can use the address as envelope sender;
    * domain postmasters can use arbitrary sender addresses under their
      domains;
    * domain owners can use arbitrary sender addresses under their domains,
      unless it is also an existing account name;
    * for known domains without owner or postmasters, other sender addresses
      are not allowed; and
    * arbitrary sender addresses under unknown domains are allowed.

Interhost communications are protected by stunnel4.  The graphs are only
visible on the master itself, and content is generated by Fast CGI.

(Unless a new instance is created, or the master.cf change is modified.)
Changing some variables, such as inet_protocols, require a full restart,
but most of the time it's overkill.