Document that the front-end POP3/SMTP listeners are unencrypted and
unauthenticated, warn that they must be bound to a trusted internal
network only, and note POP3_BIND_ADDR / SMTP_BIND_ADDR for restricting
the bind address.
Fixes#9
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>