Every message sent, to whom, and whether the provider accepted it. When a parent says they were never informed, this is the screen that settles it.
Accepted is not the same as read
The log records what the school sent and what the gateway accepted. That is an honest boundary: it proves the message left, and it does not pretend to know whether a parent read it.
Per family, per message
The history is readable against a family as well as against a send, so a dispute about one child does not require reading a report of nine hundred messages.
Where a failure came from
A message rejected by the gateway is visible as rejected, with the reason — usually a wrong number or an unregistered template. Silent failure is the worst outcome, so it is the one deliberately designed out.