CGNAT (Carrier-Grade NAT) is a technique used by ISPs to conserve IPv4 addresses. Instead of assigning a unique public IP to every customer, the ISP places many customers behind a shared NAT, then gives them all addresses from the reserved 100.64.0.0/10 range (RFC 6598) on their internal network.
This means traffic from multiple customers exits through a small pool of public IPs - a second layer of NAT on top of whatever NAT the customer's own router does (hence "double NAT").
By default all internal HTTP requests are now blocked, unless mailpit is started with the `--allow-internal-http-requests` flag (env `MP_ALLOW_INTERNAL_HTTP_REQUESTS=true`).
