Firewall that allows only smtp

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http://serverfault.com – I have a network that at the moment is isolated from the internet. We are going to connect it to an existing adsl line that has a firewall,but we want to add another firewall behind it as we do not have total control over the existing firewall. The only thing we want to allow is smtp access for one computer. Can we then just put in the new firewall and only open up for port 25 outgoing? The firewall that I want to use is a POWERBIT SAFEGATE FIREWALL IX78 is this an acceptable choice to use? (HowTos)