421 Unexpected failure

[Lizard King]

I have a problem with my mail server . I can't seem to send or receive any e-mails. When i try send an e-mail to a domain on my server i receive the following error.

This message was created automatically by mail delivery software.
A message that you sent has not yet been delivered to one or more of its recipients after more than 24 hours on the queue on server01.domain.tld

The message identifier is:     1MMiuu-0005JM-RA
The subject of the message is: 
The date of the message is:    Fri, 3 Jul 2009 16:34:56 +0300

The address to which the message has not yet been delivered is:

  mert@gokceimam.com
    Delay reason: SMTP error from remote mail server after RCPT TO:<mert@gokceimam.com>:
    host mail.gokceimam.com [188.40.51.146]: 421 Unexpected failure, please try later

No action is required on your part. Delivery attempts will continue for some time, and this warning may be repeated at intervals if the message remains undelivered. Eventually the mail delivery software will give up, and when that happens, the message will be returned to you.

Does anyone know what can be the cause for this ?

I followed Falco's The Perfect Server - CentOS 5.3 x86_64 [ISPConfig 3] tutorial and didn't face any errors during installation steps