Hostname property provides you with name of user's
originating host. Using this value you can filter incoming
connections and allow only specific IP range or domain that
are allowed to connect to your server. Once user connects
and Connecting
event is fired, Hostname is already filled with information
and you can test it against some filter of your choice.
Although Hostname property is expected to be readonly,
it's not. You can write your own value into it in case
it will provide easier accessing them later on. For
instance, if you want to reference range of IPs to some
name of your choice, you can set it up to 'friends'
during Connecting event, and then just test against this
value later, or add 'on-the-fly' new messages to
their mailboxes.