James,

Disabling Internet access on the machine could have some negative affects. For example, Xeams goes out on the Internet when:

• It needs to pulls a new filtering rule. It checks for new rules every 2 hours.
• Checks for new version updates
• It needs to pulls a list of IP addresses belonging to Office 365 from Microsoft's site
• Blacklisted URLs

Therefore, blocking the entire Internet access is only recommended if you are not getting any emails from the Internet and are using Xeams strictly inside your LAN. If all you want is to restrict in-house users from sending outbound emails to the Internet, blocking port 25 is a better option.