Multistreaming allows you to archive multiple mailboxes in parallel.
User Mailbox and Journal Mailbox: Each access node that is associated with the Email Archiving App can have a maximum of 20 streams for each job. However, one service account can only use a maximum of 15 streams regardless of the value set for the subclient. If you need to use a higher number of streams, use more than one service account.
Procedure
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In the navigation pane, expand Solutions > Exchange Apps, and then click the Email Archiving App. 
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On the upper-right corner, click on Settings. 
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On the page that appears, in the General area, click Edit. The General dialog box appears. 
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In the Max streams box, enter the number of streams that you want to use. If you do not want to archive multiple mailboxes in parallel, enter a value of 1. 
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Click Save.