changetype when create subscription

Question

Hello, I created a workflow where every new mail and sent mail is notifying my server using subscriptions. Now I receive 2 notifications for each new email. One is with the keyword created and the other one updated. Otherwise the mail itself when downloading the MIME content has no difference. Now I'm wondering what exactly the difference between those 2 changetypes is. When will created be triggered and when updated. I'm thinking about just listening to change type created only but scared that I will miss out on changes of mails. Regards