Restricting information emitted

Some sites will have special, heightened concerns regarding emitting information about their internal user names, host names, etc.. There are various ways that information may "leak out"; some ways of controlling such information leakage are discussed here.

See also:
 * SMTP probe commands
 * Internal host names in Received: and Message-Id: header lines
 * Extra concerns for address canonicalization
 * The MTA