If you disable this checkbox, you're essentially disabling all unified messaging for users assigned to that particular UM service account. Messages will still be left locally on the server and can be checked via the phone, Jabber, etc.
Some details about the implications of enabling/disabling this checkbox:
Synchronize Unity Connection and Exchange Mailboxes (Single Inbox)
If you want Unity Connection and Office 365 mailboxes to be synchronized for users who are associated with this unified messaging service, check this check box.
If you enable single inbox here, you can still disable it for individual users when you configure unified messaging for Unity Connection users. However, you cannot enable single inbox for an individual user if you disable it here.
If you disable and later re-enable single inbox for this unified messaging service, Unity Connection resynchronizes the Unity Connection and Office 365 mailboxes for the affected users. Note the following:
If users delete messages in Office 365 but do not delete the corresponding messages in Unity Connection while single inbox is disabled, the messages will be resynchronized into the Office 365 mailbox when single inbox is re-enabled.
If messages are hard deleted from Office 365 (deleted from the Deleted Items folder) before single inbox is disabled, the corresponding messages that are still in the Deleted Items folder in Unity Connection when single inbox is re-enabled will be resynchronized into the Office 365 Deleted Items folder.
If users delete messages in Unity Connection but do not delete the corresponding messages in Office 365 while single inbox is disabled, the messages remain in Office 365 when single inbox is re-enabled. Users must delete the messages from Office 365 manually.
If users change the status of messages in Office 365 (for example, from unread to read) while single inbox is disabled, the status of Office 365 messages will be changed to the current status of the corresponding Unity Connection messages when single inbox is re-enabled.
When you re-enable single inbox, depending on the number of users associated with the unified messaging service and the size of their Unity Connection and Office 365 mailboxes, resynchronization for existing messages may affect synchronization performance for new messages.
You can use the "Accept and Relay" Message Action which will send a copy of the voicemail to a configured email address via SMTP and also leave a copy on the server. Keep in mind though that these voice messages do not actively sync between your client and the server like Unified Messaging offers. You'd have to manage messages in Outlook as well as in CUC. Be sure you have a message aging policy configured or you will potentially fill your disk if people don't delete their voicemail. MWI will only work when manipulating the message on the server locally, anything done with the SMTP relayed copy is unknown to Unity (read, deleted, saved).
