[user5994461]

Never worked in a bank? Never worked in defense?

I'm speaking from real experience too. It takes a while to open firewall in some environments, if you ever can.

One bank was the worst. There was a super stringent process to expose things externally. Opening the firewall port was just the beginning and that'd take 2-4 weeks if all goes well.

You'd struggle like hell to expose a SMTP server though because it would be immediately be rejected and flagged based on the port. Banks have to store, monitor and ensure the origin of all emails, they don't allow shadow email servers. And it's plain text so more reasons to ban (also a problem with HTTP, you should do HTTPS if anything).

Defense was simpler, mainly because there was no external connectivity in many cases. You don't need to worry about how to open a firewall when there's none :D