Receipt messages are just the Exchange Return Receipt emails, every time that you leave a VM to another subscriber, and that subscriber checks it, you will get a return receipt VM message to let you know that that subscriber already checked your VM. This happens if you have this option enabled on Unity.
There is a setting that was included on an Unity version that avoids treating return recipes as new voicemails, you should check the Advance Settings Tool on the Cisco Unity Tools Depot in order to disable the notification.
Check these links:
http://cisco.com/en/US/partner/products/sw/voicesw/ps2237/products_tech_note09186a00800cdd7b.shtml
http://www.ciscotaccc.com/kaidara-advisor/voice/showcase?case=K30147579
