currently we don't encrypt DMs to 3PID invited users, as they don't have an E2EE identity yet. But we should check that E2EE gets turned on once they onboard and the DM doesn't get left arbitrarily unencrypted...
Presumably this should be done by the receiver given that the sender may not be online at the time, pretty certain nothing handles this atm on any client.
currently we don't encrypt DMs to 3PID invited users, as they don't have an E2EE identity yet. But we should check that E2EE gets turned on once they onboard and the DM doesn't get left arbitrarily unencrypted...