Update the default host from localhost to 0.0.0.0 to allow the server
to accept connections from external interfaces when no HOST
environment variable is provided.
Add logging to `sendHandler` and `sendParamsHandler` to track incoming
requests. Update the server startup logic to use `HOST` and `PORT`
environment variables, defaulting to `localhost:660` if not provided.
Add detailed structured logging to `sendHandler` and `sendParamsHandler` to track request details, domain lookups, and SMTP outcomes (success/failure). Update the server to bind to `localhost` instead of `0.0.0.0` for improved security.
Tomas Dvorak