PST migration to Exchange online

Copper Contributor

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.

2 Replies

Hi Matt887,

Great question! When migrating PST files to Exchange Online using the network upload method, the behavior of Outlook desktop clients largely depends on your mailbox caching settings. Here's a breakdown:


Online Mode:

If the Outlook profile is set to "Online Mode," it means emails are directly accessed from the server. In this case, only headers are initially downloaded, reducing the impact on bandwidth.


Cached Exchange Mode:

On the other hand, if the profile is configured with "Cached Exchange Mode," Outlook will download a copy of the entire mailbox to the local machine. This provides users with offline access to emails but might lead to bandwidth saturation during the initial sync.


Given your concern about bandwidth. I suggest to use "Online Mode" for users with high-speed internet. Stagger migration for smoother transitions. Initially set profiles to "Online Mode"; switch to "Cached Exchange Mode" post-migration.


If you are wishing to know the other methods, follow these helpful guide: PST to Office 365 Migration  and I found a helpful thread it can help you with PowerShell command while importing PST to office 365. Here is the thread to post: Import PST to Office 365 using PowerShell Command



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.