To migrate Outlook PST data to Microsoft 365, first identify the PST files and determine which Microsoft 365 mailbox should receive each file. For an individual user or a small migration, the PST can be imported through Outlook. Microsoft supports importing Outlook email, contacts, and calendar information into a Microsoft 365 mailbox, although certain Outlook-specific settings such as rules, views, permissions, and AutoArchive configuration are not transferred through the PST import.
For enterprise migrations involving many PST files, the Microsoft 365 PST Import service is generally more appropriate. Administrators can use the network-upload method or drive shipping to move PST files into the Microsoft 365 import workflow. Microsoft also provides filtering capabilities and duplicate-item handling as part of the PST import process.
A third-party solution such as the SysInfo PST Converter Tool can be useful when the migration requires additional selection and conversion controls. It supports multiple PST files, selective folder migration, date filtering, duplicate handling, attachment options, and direct migration to Microsoft 365.
Before starting a large migration, administrators should create a source-to-destination mapping, check mailbox sizes, test a representative group of PST files, and verify the resulting emails, attachments, folders, contacts, and calendars. Microsoft currently notes that Exchange Online PST imports have limits, including a 150 MB maximum message size and an approximate import rate of 24 GB per day per mailbox, although actual performance can vary.
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