The LocalDomain returns the local domain name used when the
client identifies itself to the mail server. If this property is an
empty string, then the control will attempt to automatically
determine the appropriate domain name to use based on the system
configuration. Setting this property will cause the control to use
that value when identifying itself to the server.
This property should only be set if it is absolutely necessary. In
most cases, it is preferable to leave this property undefined and
allow the control to automatically determine the correct domain name
to use. Setting an invalid domain name may cause the mail server to
reject the connection.