See https://sentry.matrix.org/sentry/synapse-matrixorg/issues/190798/

It looks like this is happening when a user is no longer in the room when prepping to send an email notification (this is likely due to the user leaving during the delay between an event being received and sending the email).

For some situations we should likely just ignore the notification and move on, but in some situations we probably want to notify the user that they've been kicked / banned / whatever.