In your environment when you have installed Exchange 2013, a new default database is created.
In same cases they are either renamed or deleted. If the mailbox database is deleted then you might get the following warning logged in event viewer rather often:
- The Microsoft Exchange Mailbox Replication service was unable to process jobs in a mailbox database.
Database: Missing database (GUID)
Error: Database ‘GUID’ doesn’t exist.
You can either restart the Microsoft Exchange Information Store or the server and the error will be gone after that.
Hope it helps.