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Matt887's avatar
Matt887
Copper Contributor
Nov 16, 2022

PST migration to Exchange online

Hello everyone.
I have a question regarding Exchange Online and Outlook on the desktop client.
Our customer wants to migrate an old pop3 web server to Microsoft 365, so I will upload all desktop client's outlook pst to exchange online. Use network upload to import PST files - Microsoft Purview (compliance) | Microsoft Learn
When PC clients open the new Outlook profile (desktop version), will they download all entire emails or just the headers?
Because the customer has a slow internet connection and if the pc clients download all the entire emails there may be bandwidth saturation.
Thanks in advance.

  • Dan_Snape's avatar
    Dan_Snape
    Steel Contributor
    Because you will need to create a new mail profile in Outlook, a new OST will be generated which contains a cached copy of the mailbox. The size of the OST is determined by how much mail is cached, and this can be controlled (you don't need to cache the whole mailbox). Using online mode in Outlook when connecting to a mailbox in Exchange Online will be a painful experience for users due to the lag when accessing message items over the internet, especially over a slow link, so I wouldn't recommend it. I would recommend configuring a small cache for each user initially (maybe 1 month or less) then expand it post migration when things settle down.

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