I had a problem when I first was installing - the installer doesn't have the <address> tag in the config, it was needed for others outside my network to reach my server.
Code:
<server>
<world>
<shortname>servername</shortname>
<longname>longservername</longname>
<address>external_ip</address>
<loginserver>
<host>127.0.0.1</host>
<port>5998</port>
<account></account>
<password></password>
</loginserver>