Six Colors explained that end-to-end encryption status can change within the same conversation when iMessage switches to RCS, SMS or MMS. This concerns changes in recipients or transmission paths, not a failure of the cryptography itself. Users should check the conversation’s notices and information screen for the current status; SMS and MMS do not provide end-to-end encryption. Disabling fallback delivery can prevent some messages from arriving, so security and deliverability must be considered together.