Migrating from Outlook Classic to Outlook 365

Hi Michael Villanueva

I understand how confusing it can be to see emails missing after moving to the new Outlook. Have you already tried the Q&A Assist suggestions? If yes, what happened?

In addition, please check whether your missing emails exist only in a local PST file in Outlook Classic.

If you haven’t already, also sign in to Outlook on the web and verify whether the missing messages are visible there. If the emails don’t appear in web mail, they’re not on the server and would confirm they were stored only locally.

In Outlook Classic:

• Go to “File” > “Account Settings” > “Account Settings”
• Open the “Data Files” tab
• If you see an Outlook Data File (.pst) listed and set as default, emails stored there will not appear in the new Outlook

If your missing emails are in a PST:

• Open Outlook Classic
• Move or copy the needed emails from the PST into your main mailbox folders (Inbox or custom folders under your account)
• Allow time for sync
• Open the new Outlook and check again

Please feel free to let me know how it goes. I truly appreciate your time and understanding.

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Not all emails migrate when moving from classic Outlook to Microsoft 365/Outlook 365 for a few common, documented reasons:

1. Account type / sync configuration is wrong
   If classic Outlook was using one account type and the new Outlook/Microsoft 365 profile is using another, some folders or historical mail may not appear.
   • For example, a Hotmail.com or Live.com address must be configured as Outlook.com account type, not Exchange or Microsoft 365.
   • When the account type is wrong, classic Outlook “isn't syncing my messages and folders,” so only part of the mailbox appears in the new profile.
2. Using IMAP migration or IMAP accounts (email only, no contacts/calendar)
   If the move involved IMAP (either via Outlook or via Microsoft 365’s IMAP migration tools), only mail folders are migrated:
   • IMAP migration does not migrate contacts, calendar items, or tasks—only items in Inbox and mail folders.
   • There is also a limit of 500,000 items per mailbox and a 35 MB maximum size per email. Items beyond these limits or larger than 35 MB will not migrate.
3. Policies or archiving on the source system
   If the old environment had messaging records management (MRM) or archival policies:
   • Messages that were auto-deleted or moved to archive by those policies are flagged as “missing” during migration.
   • This can look like data loss even though the items were moved or deleted by policy before or during migration.
   • Microsoft recommends disabling MRM and archival policies before migration to avoid this confusion.
4. PST-based moves vs. server-side migration
   If email was exported from classic Outlook to a .pst file and then imported into a Microsoft 365 mailbox:
   • Only what was actually exported into the .pst will appear in Outlook 365. Anything not included in the export (e.g., excluded folders, archives, or large items) will not show up after import.
   • To import a .pst into a Microsoft 365 mailbox in classic Outlook:
     1. In classic Outlook, select File → Open & Export → Import/Export.
     2. Choose Import from another program or file → Next.
     3. Select Outlook Data File (.pst) → Next.
     4. Browse to the .pst, choose how to handle duplicates, then Next.
     5. Select the Microsoft 365 mailbox as the destination and choose Finish.
5. Autodiscover / profile issues after migration
   After a mailbox is migrated to Microsoft 365, Outlook can still be pointing at the old server (Autodiscover cached in the profile). Symptoms include:
   • Outlook cannot connect properly or shows incomplete data.
   • Web services (online archive, availability, etc.) do not work.
     In such cases, creating a fresh Outlook profile that connects directly to the Microsoft 365 mailbox (with corrected Autodiscover) is required so that all server-side mail and folders sync.

If specific folders or date ranges are missing, check:

• That the account is added with the correct type (e.g., Outlook.com vs Exchange vs IMAP).
• Whether IMAP migration limits (item count/size) or policies might have excluded some messages.
• That all needed folders were included in any .pst export/import.
• That a new Outlook profile is created against the Microsoft 365 mailbox if the environment was migrated.

References:

• Troubleshoot Outlook email setup
• What you need to know about migrating your IMAP mailboxes to Microsoft 365 or Office 365
• Import email, contacts, and calendar from an Outlook .pst file
• Outlook doesn't connect or web services don't work after migration to Microsoft 365