I've raised a PR to allow cancelling emails in the queue - itflow-org/itflow892
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Reviewing the latest ticket scheduling code, it seems that schedule confirmations are sent to contacts even when no contact is set / the contact doesn't have an email. I don't have time to unpick the code, but I would recommend we separate out the $data array to allow the emails to contacts, the agent and watchers to all check for valid emails.
https://github.com/itflow-org/itflow/blob/da54b125c3e6cc377288db01810c80ce3b1dd8c6/post/ticket.php#L1528C14
https://github.com/itflow-org/itflow/blob/da54b125c3e6cc377288db01810c80ce3b1dd8c6/post/ticket.php#L1672