After this mailserver (SMTP/POP) is installed on one computer,
users can create new accounts and send mails
to other users of the same server. If you
want to allow them to send mails to users of other
servers you must have a static IP address and configure
a SMTP relay server. Otherwise you do not need to
press the button "SMTP configure" and change the values.
The reason for this limitation is called dynablock
which is installed by nearly every internet provider.