If a quiz is set to have a grace period when time has expired, we only
email studnets to warn them about their overdue attempt if they have put
in an answer to at least one question that is worth some marks.
This avoids the problem that you cannot send messages in transactions.
It also means that the quiz submission will not be prevented, and the
message will still be sent eventually, if any part of the messaging
system is giving intermittent errors when the student wants to submit
their quiz.
Tim Hunt