connection timed out

Hi,
Usually, I have no problems connecting to a site (for instance, no problems connecting to ftp.gnu.org), but this one maybe wants me to do something special.
This is the message I'm seeing in Filezilla:

Status: Waiting to retry...
Status: Resolving address of address1.address2.nl
Status: Connecting to 12.34.567.89:21...
Status: Connection established, waiting for welcome message...
Error: Connection timed out
Error: Could not connect to server

The FTP client from Total Commander 8.01 gives me a socket error 10054.

I'm behind a NAT router.
Can you help me please?
Thanks,
Xpoes