Just search on the forums. Several people had this problem before.
I think your /etc/hosts isn't correct. What do you have in there? It should look like:
Code:
127.0.0.1 localhost.localdomain localhost
192.168.1.157 mail.example.com mail
The first line exactly like this. The second line with the IP adress, the FQDN and the hostname of the machine.