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I’d say, no, it isn’t really necessary. A solid inbound firewall is obviously important, but outbound firewalls have little demonstrated benefit that I’ve seen. I guess it could help prevent your machines from spewing out spam (which means you have other problems if you don't work at Spam R US), assuming the outgoing firewall could distinguish between the spam and legit traffic. Others may disagree. Here is an article on the topic that you might like: