Syncing issue with only one Mailbox on Exchnage 2010 to office 365

I have migrated a SBS server Exchange to office 365. All the mailboxes have synced but one of them. I want to do a export to PST but can't do it via outlook . Is there another way of doing this so that I can then export the past to office 365. The mailbox is around 6 GB.
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netcompAsked:
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SaberSupport Escalation EngineerCommented:
This article walks you through step by step on how to do an Export to PST using Exchange Management Shell.

https://blogs.technet.microsoft.com/sbs/2009/01/13/sbs-2008-how-to-export-and-import-mailboxes-to-and-from-pst/
Tom CieslikIT EngineerCommented:
Yes, you can convert your existing OST file in user profile to PST file then import it to O365
http://www.ost2.com/
Ajit SinghCommented:

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Ajit SinghCommented:
No further questions from author. Please post back if you need any assistance.
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