0x800ccc0f Suse 9.3 qmail vpopmail 5.4.17

Hi i have configured a qmail box using vpopmail 5.4.17 in a SuSE Linux
9.3 (i586) - Kernel 2.6.11.4-21.14-smp (0) machine.

Everything was working fine, but i had to change the ip address of the
server since then some of my clients can't connect to the server and
receive the 0x800ccc0f error.

I have changed the ip address again, trying to change the MTU but they
were already at 1500 and i can't change it to a higher value.

I don't think is an outlook problem, because my outlook was working
fine, but then after the change it stopped working.

Does anyone has any suggestion about this problem?

Thank you very much.

I Forgot to add that this server is not behind a firewall and is not
using proxy