Recently we have been seeing several issues in Support Services where Event ID 623 is being logged on the server. The symptoms include "Users unable to access their inbox", "Email backing up in the local delivery queue" and "Email awaiting directory lookup". Clients may also see emails stuck in the Outbox. Rebooting the server usually fixes the issue. In this blog, I have tried to explain what "Version Store" is and how to troubleshoot this issue.
A transaction is a series of operations that are treated as atomic (indivisible). So the operations in a transaction are either all done and permanently saved, or none of them are done. As an example we can consider the operations involved when we do something very simple like move a message from the inbox to the deleted items folder. The Version Store gives ESE the ability to track and manage current transactions; this allows ESE to implement Isolated and Consistent transactions.