PST migration to Exchange online

[Matt887]

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]

Hi, @Matt887 

Aryson Import PST to Office 365 Tool can easily migrate single or multiple PST files into Exchange online without any email data loss. The demo version of this software is totally free, you can use and migrate up to 50 items per folder without any cost.