The answer is "it depends". First, read the two sections of the Unified Messaging Guide for CUC regarding the interaction between CUC and Exchange/O365:
Synchronization with Outlook Folders and Location of Deleted Messages
"Reading" a message in Outlook will cause the status of the message in CUC to change from "New" to "Saved". Deleting a message in Outlook will cause the status of the message in CUC to change to "Deleted", which places the message in deleted messages in CUC.
You'll have to look at the Message Aging Policy to see if and when messages are purged from Deleted Items.
Maren