This happens because of how SmartActions work in K2 Five. Due to the removal of MSMQ in K2 Five, the primary email account that is used for SmartActions will now go and look at Exchange and all the connected accounts in correlation to it.
This is the reason why a great amount of emails could be triggered to be sent out. To resolve this, SmartActions will need to have a primary account with its own mailbox that is only used for its purpose and not an account that is used to send emails to and from. For example, an administrator account will not work as expected and more emails will be sent out.
To resolve this issue, create a primary account and switch with the current account that is being used for Exchange. Alternatively, create a new server with the correct primary exchange email account for SmartActions use.
Please also see: https://help.k2.com/onlinehelp/k2five/userguide/5.0/default.htm#Use/SmartActions/K2_SmartActions.htm