When you're in the Message Center, click on the Message History tab to view a list of every message sent. You can see who sent the message, the subject and body of the message, and the number of accounts it was sent to.
Click on the accounts link to see a list of accounts that received a message. The viewer will only see the names of accounts to which he or she has access. If there are other accounts that received the message, the viewer will know the number of accounts, but won't see any information about those accounts.