How to see the phone numbers a message was sent to? I sent a message to several people, and one of them indicates that they never got the message. Is there a way that I can email a copy of the message showing all the phone numbers the message was sent to?
No.

The iMessage function will send delivered and read receipts for messages delivered by Apple's Push Notification Service, but not through the carrier's SMS network.

You would have to send each message individually if you wanted to know if each message is received, but that also depends on the recipient opting in to share read receipts. If their devices are not configured to send back that confirmation, you need a mechanism that is outside the band of Messages to know a message was received. (like calling them, asking, sending a link that tracks cookies and they identify themselves in this interaction, etc...)