[Error Code] 0x80040305 [Explanation] 
1. This is caused by maximum number of open objects (folders) that an Exchange server can allow within a session. All the users are processed under the same session so all the open folders for all the users can add up. 2. The number of open folders for a user depends upon the maximum number of children folders under a parent folder. So if we have a folder tree where in we have max 10 children under a parent then we would not hit this limit even if user has thousands of folders in total, but if user has 500 folders under 1 single folder then we will hit it. It really depends upon the folder distribution. 3. Doing PST migration will really help for such user. Because open folders will not consume Exchange resources and will not count towards the limit and also we can choose to do a partial PST export. To resolve the issue without having to change the folder structure, please try migrating the user with the GAMME PST option. Alternatively, follow the steps outlined here to increase the number of default value for the number of open objects that are allowed.