Cannot Authorizing from Outlook Express (POP)

[smartech]

This is the first time im using zimbra for email server. In installing process, there is error when ask to creating domain. i skipped this process.

Then, the email server done. there is no problem at first trial between 2 computers. I try to pop email from email server to my laptop using Outlook Express, and it works. I try to send email from web based by using 192.168.0.1 (example), and its works smooth.

Later, i try to connect it to larger network with 10 computers. All of them, excepts my laptop, cannot pop from outlook express, but able to check,compose, and sending email to local via web based.

My server configuration :
ip : 192.168.0.1
hostname : mailserver
domain name (bind9) : smalloffice.com

My Client configuration :
ip : 192.168.50
hostname : user50
email account : purchasing@smalloffice.com
password : easypassword
Incoming mail (pop):192.168.0.1
Outgoing mail (pop) : 192.168.0.1
My servers requires authentifications checked
Always connected with Loca Area Network
Pop :110
smtp: 25

Is this caused by the error when creating domain ? how to fix it?
Thanks for helping