http://7fa6xlti73nrpxl6cykao4wp7g3bg4624ghrtdomlvkeivvwclv2iyad.onion/anonbib/cache/rewebber.html
Second, type II remailers use constant-length messages, to prevent
passive correlation attacks where the eavesdropper matches up incoming and
outgoing messages by size. Third, type II remailers include defenses against
sophisticated replay attacks.