If you're using Outlook Email client application where all your email saved in PST or Personal Storage Table file format then these two thechnique Split and Merge will definitely help you to come back from two major issues:

1. When have large size Outlook PST file.
2. When have large number of Outlook PST file in Outlook account to access.  

Split Technique: This technique will help you to break large size size Outlook PST file in some small PST files so that user can use data of large size file easily. 
Merge Technique: It will allow user combine and merge large number of Outlook email files in one single file.

You can take more information about both two technique from the link: Split and Merge Outlook PST file