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`).
- Updated error handling to use the error return value for resource closures in tests and functions, ensuring proper error reporting.
- Replaced direct calls to `Close()` with deferred functions that handle errors gracefully.
- Improved readability by using `strings.ReplaceAll` instead of `strings.Replace` for string manipulation.
- Enhanced network connection handling by adding default cases for unsupported network types.
- Updated HTTP response handling to use the appropriate status codes and error messages.
- Removed unused variables and commented-out code to clean up the codebase.
The module microcosm-cc/bluemonday now requires Go v1.21 and is quite frankly an overkill as Mailpit only needs to convert HTML to a single line (no formatting).
