[courier-users] Mail sub domain

[shyam s]

Dear Group.

                I had another issue. I created a web site called www.postbox.com, let me say it have the ip address 192.168.0.10 . With a dns server running on it. I need to setup a mailserver called mail.postbox.com  which must be running on a remote system(same network) and its IP number must be 192.168.0.20.

So in the postbox.com{in its dns} , I had made the MX bit as 192.168.0.20 so when I type

[EMAIL PROTECTED] root]# ping mail.postbox.com  it is working properly.

I had add /etc/courier/hosteddomains  the entry mail.postbox.com

My login id must be [EMAIL PROTECTED],for this I am using mysql authentication.and it works. I can send to mail to my other domain to the mail id [EMAIL PROTECTED]. But the real problem is when I make a reply from [EMAIL PROTECTED] it wont reach the [EMAIL PROTECTED] and it will remail in the mailque. When I check the log it shows the error  in /var/log/maillog

450 Service temporarily unavailable.

What I understand is that when the mail is sending from [EMAIL PROTECTED] to [EMAIL PROTECTED],courier will be searching for the domain postbox.com which orginally have the ip 192.168.0.10, and it fail to delever the mail since my actuall smtp is running on mail.postbox.com which is running on 192.168.0.20.

In this situation how could I make this kind of approch to work. Is there any kind of aliasing possible. Please do correct me, actually I am very new in mailserver/dns configurations.

Thanks and Regards
Shyam