Cameo reminder messages now show who was sent a copy by email. Find this under the text of the message in Library > Messages (Fig 1).
Cameo users subscribe to the tags assigned to messages to receive email. This means all, some or no messages may trigger email copies, so you may not have known whether someone has seen a message or not.
Of course, sending an email doesn’t necessarily mean they read it, but at least you know they should be aware.
This is not retrospective: only messages emailed after 9 December 2000 carry this information.
Administrators can also see which tags people subscribe to in Admin > Cameo users.