The K2 Server periodically checks the Exchange mailbox that has been set up for SmartActions for new or changed messages, based on the Message Sync State which is set by Exchange. If this mailbox is moved from one Exchange server to another (typically Exchange 2010 to Exchange 2013) the Message Sync State becomes invalid. When the K2 Server checks the mailbox on the new Exchange server, all messages are regarded as new and are re-sent. This can have a impact on network and server performance as well as duplicate messages being sent to users.
Before moving the mailbox associated with SmartActions, any messages need to be moved or deleted from the Exchange inbox. The loss of the Message Sync State then becomes irrelevant as there are no SmartAction messages to be re-sent.