FX is referred to as “Fast Transfer”. Internally properties are kept raw formats and dynamically converted to meet the specification of the requester. Using Fast Transfer, the properties of a message can be copied in their raw format without translation. This object represents a source stream when copying properties.
Open Item Limits
Open item limits are limits placed on the number of items that can be opened by a single mailbox in a single session. However, a user can have multiple sessions opened simultaneously. For example, if a user has two sessions opened, the user could open 1,000 folders.
If you want to modify these limits, see "Configure Open Item Limits" immediately following the table.
Did you check out this forum post?