Why can't some one join into my server?

1/29, I let tiki join into my server(1). but he/she can't join my server, Why?
When he/she was joining, It pops TIMED OUT! Very crazy, isn't it?
1/30, I let marblejc join my server, he/she says:"I can't join your server, it says TIMED OUT." Very crazy, isn't it? isn't it?!
Aren't you often encounter this situation?
(1) It refers to HYZ's Server.

You need to Port forward port 28000.
Technically you can set Marble Blast to any port which isn't 28000, e.g. 28001 (I've done that for a few weeks during development), but you have to change your mbpprefs.cs to the new port as well. Otherwise other users cannot join you.

You can see if you port forwarded successfully by going to the server settings and seeing the server status. If it says LAN Only, it means you haven't port forwarded. If it says Successful, then you're good to go.