Connection time out to certain email domains
We have Zimbra 5.01 (OS) installed on an Ubuntu 7.10 server. All has been working fine up to last week. I have four email domain address' that now time out and are constantly in the defered queue on the server. I can ssh into the zimbra/ubuntu server and telnet hostname 25 but cannot send mail via email client or web access. If I wait long enough I do get a delivery failure but only that it could not send.
I am a slightly higher than linux newbie. What should I be looking for? This machine has a static outside address via firewall NAT.
Thanks in advance for any help.