I am new to the linux enviroment and require help.
We have an apple mail server, but recently we are receiving a lot of spam and have other issuses with the server.
So is it possible to put a linux box before the the mail server, which will download the email from the internet for addressed to a specific domain. Once the emails are downloaded, forward the emails to the Apple server for distribution.
For Smtp it should only accept emails from one single domain name or an IP address and send it to the internet.

Thanks in advance

your linux box could easily act as your mail server and then filter out the spam. If you wanted to, it could then pass along the mail to your aple box for distribution or whatever to the lan. If i was going to do thid, however, i would either a) just use a mail filter on the apple box. or b) change over to the linux mail server completely. But, what your looking for is completely possible.

what do i need to configure to setup the mail server to receive emails from the interent and forward the mail to the apple server

check out http://www.sendmail.org/compiling.html , also looking at the sendmail config file gives you an almost step by step config. Some may argue, but i found it to be very intuitive