How can an ISP be sure an end-to-end encrypted communication channel isn't being used for porn?
It is encrypted. By design they have no idea what it is being used for. It would be very easy to set up a ring of pornography distributors using email (at least in the UK, in civilised countries they'd probably be put out of business by the open internet).
By this interpretation, any TLS channel, even to a supposedly "clean" domain, can be used for porn. Btw, nothing prevents you from emailing porn over Gmail or any of the other "allowed" email providers.
It is encrypted. By design they have no idea what it is being used for. It would be very easy to set up a ring of pornography distributors using email (at least in the UK, in civilised countries they'd probably be put out of business by the open internet).