I have recently installed a exch.server for our office. we were using webmail option for all our staff proivded by our webhosting service provider. After setting up the exch server, I have asked our webhosting co to point our mail.company.net to our public ip (eg.188.8.131.52). After that I can send email from exchange based outlook clients but cannot receive any. How to I set our public IP to point to our exch server machine ?
Mail is transferred between servers using SMTP, Simple Mail Transfer Protocol. Well, there are others, but not relevant here and now. I think you'll be fine if you simply port forward through your router. Port 25 is the default SMTP port. So if you forward port 25 to the Exchange Server's internal IP, I think that'll work.
NOTE: right now, you're basically forwarding everything from your webmail account to your Exchange server. If you intend to cancel that webmail service, then eventually, you're going to need to change the MX records for your public domain. Generally, most domain registrars have customer support staff that can get this taken care of for you. My company has taken control of the DNS records for our public domains, and I can edit those MX records myself, should I so choose.
Changing those MX records would direct email directly to your Exchange server, bypassing your webmail provider completely. And I think the port 25 forwarding would still work, although Exchange has a component (whose name escapes me right now) that can be installed on a PC with a public IP and send the email traffic into the message store from there.