Migrating a client from FuseMail hosted Exchange to Office 365. Unfortunately, FuseMail has no administrative controls, and the client only has 8 user accounts, so I've fired up some virtual machines and installed Windows 10 Pro along with Outlook 2016 and connected each of the accounts to their own individual Outlook client. I configured each and every client to cache ALL data from the server, yet I see massive discrepancies in the OST compared to the Mailbox size in the admin panel for Fusemail. I understand these numbers can be different, but for example, I have a user that has 32 GB of account data in FuseMail, however the current OST size after all folders are up-to-date ticks in at a measly 3 GB.

Alarmed, I compared the earliest email data on the Outlook client to that of OWA in this particular account, I scrolled down to the bottom of the Inbox and to my dismay, noticed that there was a "There are more items in this folder on the server" message. I double-checked the configuration of the Outlook client and verified that it is indeed set to cache ALL data (as opposed to 1 year, 6 months, etc). I then noticed that ALL of the Outlook clients are the same -- single-digit OST files in the GB, whereas double-digit mailbox data for the same respective account on the server. In addition, even after clicking the "Show More Results" link, the oldest message I can find in the Outlook client is 2 years old, whereas in OWA on the server, the oldest message is 4 years old!

I want to move this data as efficiently as possible. Is there any way to configure Outlook to force it to download ALL data to the OST so I can export it to PST for the migration? Could this be a limitation coming from the FuseMail Exchange host somehow?

Any help would be greatly appreciated.


Do you configure FuseMail mailbox in Outlook as POP3 or IMAP4? You said "OWA", does it configure in Outlook as Exchange mailbox? If so, select "Mail to keep offline" and select "All". enter image description here

Since, it's an Exchange mailbox, we can use New-MailboxExportRequest to export content to .PST file.

By the way, we can do mailbox migration directly to Office 365 by Stage, Cutover or Hybrid.

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