Outlook has 2 types of data files – OST and PST. The OST file is the offline copy of the emails stored on your computer. It can be synchronized with Outlook when connected to the online server. Also, it allows users to check emails when the computer is offline. The maximum size of the OST file is 50GB for the latest versions. The maximum size of the Outlook mailbox folder is 100GB. You can add both OST and PST files in this space.