I think what I need to do here is two queries and union the two.

There are two scenarios, a message from one sender to one recipient.

A message from one sender to multiple recipients.

These are essentially two messages since the latter, while being sent to multiple recipients, is the same message, duplicated to multiple recipients.

When producing a list, I don't want to show multiple copies of the same message. Imagine, if you used gmail, and instead of showing grouped messages, it displayed a copy of each one. Its not an ideal presentation of that information.