ok before this my company have mail server base at US and my company base in Malaysia! and now they wanna take back the mail server here.

So what i need to have for build new mail server here?? actually im not familiar with mail server actually.

Any suggestion on what mail server is the best?? (open source)

There aren't as many threads here on "which mail server" as "which distro", but there are still plenty if you want to do a search.

The four you will see recommended are:

sendmail
postfix
exim
qmail

You'll also get the odd recommendation for one of the packages like zimbra.

Personally I use postfix, since it can do most things you'll need, is easy to configure and secure.

You'll also need an imap/pop server like dovecot or cyrus, and perhaps webmail like squirrelmail.

Knowing your distro would help, because RH, for example, supports sendmail, postfix or exim out of the box with dovecot the preferred pop/imap server.

is postfix have webmail?? have GUI configuration?

postfix is an MTA (mail transfer agent) which is a different thing. A typical webmail (like squirrelmail) interfaces with an imap server (like dovecot) and an MTA (like postfix).

I believe you can configure postfix with webmin, but mailservers are generally more granular that suits pretty gui interfaces (plus most mail servers don't use guis anyway).

I would look into Citadel, http://www.citadel.org/. Works out of the box most of the time, provides virtual users or host-based users, gui configuration, webmail, groupware, chat, calendar, and it is a nice collaboration environment. Spamassassin and clamav anti-virus can be added in if needed. It is very easy to setup. Postfix and others are great mail servers but require extensive work to gain the same end results as Citadel, and in the end you will have a much more cohesive setup using Citadel.