Create a public folder in Microsoft Exchange Server to fix the issue
This article applies to:
Any attempt to back up an Information Store fails with the following error message:
MAPI error 1,318: The Microsoft Exchange Server computer is not available.
Either there are network problems or the Microsoft Exchange Server computer is down for maintenance.
The storage group that you are trying to back up does not have a public folder.
Create a public folder for the storage group in Microsoft Exchange Server to fix the issue.
See Microsoft TechNet Article: How to Create a New Public Folder Database
Feel free to contact Acronis Customer Central if you have further questions or need assistance.