How does EM Client store emails locally ( or what is the default setting ?)

I have set up EM Client at home and set up my email account from work. We use Microsoft Exchange. So far so good, it seems to have pulled in all my emails and I am particularly impressed with the calendar.

However when I click on some emails there is a lag in getting the email. I am used to Outlook with the ".pst"file or in Exchange I think it is called ".ost"which I believe stores the information locally

Does EM Client store all the emails locally or for some old emails does it act like a web browser and only retrieve the email if I click on it?

I went to the following folder C:\Users\Andrew\AppData\Roaming\eM Client and the folder was only 450MB yet I know my exchange account is sitting at 4GB  

I am quite happy if it does not pull in all emails, save some bandwidth , but I would like to know how it works. Thanks