Email Migration

How to Fix IMAP Mailbox Migration Authentication Errors in Microsoft 365

Aamir Naveed Akram Oct 20, 2026 870 views

Question details

The user is experiencing an authentication error when attempting an IMAP mailbox migration to Microsoft 365, despite the account functioning correctly in desktop email clients.

Fix IMAP Mailbox Migration Authentication Errors in Microsoft 365

Product: Microsoft 365 / Exchange Online
Device & OS: not provided
Scenario: Migrating IMAP mailboxes from a third-party host (such as Bluehost) to Microsoft 365 using a CSV batch migration file in the Exchange admin center.
Observed behavior: The migration batch fails to authenticate against the source server because the uploaded CSV file contains only the local account name rather than the complete source email address.

Before you start

Ensure you have Exchange Administrator access in Microsoft 365 and have the exact source email addresses and passwords ready before updating your migration batch.

Solution 1Recommended

Use Full Email Address in the Migration CSV

Update the migration CSV file to ensure the UserName field contains the complete source email address, which allows Microsoft 365 to successfully authenticate with the source IMAP server.

A common mistake during IMAP migrations is using only the local account name (the part before the @ symbol) in the migration file. While some local clients like Outlook can auto-resolve this, the Microsoft 365 migration endpoint requires the full, explicit email address to connect to third-party hosts like Bluehost.

Open your migration file

Launch a spreadsheet editor such as WPS Spreadsheet or Microsoft Excel, and open the CSV file you are using for the migration batch.

Update the UserName column

Locate the 'UserName' column. Replace any isolated local account names with the complete source email address (e.g., change 'john' to 'john@yourdomain.com').

Verify remaining columns

Ensure the 'EmailAddress' column contains the target Microsoft 365 routing address and the 'Password' column contains the correct password for the source email account.

Save and upload

Save the file in CSV format. Go to the Microsoft 365 Exchange admin center, delete the failed migration batch, and create a new batch using the corrected CSV file.

Use Full Email Address in the Migration CSV

Format Integrity: Always double-check that your spreadsheet program does not inadvertently alter the CSV formatting or drop leading characters when saving.

Solution 2

Verify Modern Authentication Requirements

Check if basic authentication deprecation is impacting the connection to your source email server.

Free Microsoft Office alternative

Prepare Migration CSV Files Safely with WPS Office

Editing migration batch files requires a spreadsheet editor that maintains strict CSV formatting. WPS Office is a lightweight, fully compatible alternative to Microsoft Office that lets you manage your migration data effortlessly and completely free of charge.

1. Download and Install: Get WPS Office for free from the official website and install it on your computer.
2. Open CSV Safely: Right-click your IMAP migration CSV file and select 'Open with WPS Spreadsheet'.
3. Edit and Save: Update the full email addresses in the UserName column and easily save the file while retaining pure CSV encoding.

Fully compatible with Microsoft Excel CSV, XLS, and XLSX formatsPrevents unwanted data formatting changes when saving CSV filesFamiliar, clean user interface requiring zero learning curveLightweight architecture opens large data lists instantly

microsoft office alternative - wps office

Frequently Asked Questions

Why does my IMAP account work in Outlook but fail during Microsoft 365 migration?

Outlook and other desktop clients sometimes auto-resolve or cache local account names for incoming server configurations. However, the Microsoft 365 cloud migration endpoint requires explicit authentication and needs the full email address to authorize properly against the source IMAP server.

What headers are strictly required for an IMAP migration CSV file?

Your CSV file must contain exactly three column headers for a standard IMAP migration: EmailAddress (the destination Microsoft 365 mailbox), UserName (the full email address of the source mailbox), and Password (the password for the source mailbox).

Can I migrate emails directly from Bluehost to Microsoft 365 using IMAP?

Yes, Bluehost fully supports IMAP connections. You can use the Microsoft 365 Exchange admin center to perform an IMAP migration by configuring an endpoint pointing to Bluehost's IMAP server and providing a properly formatted CSV file with the complete source email addresses.

Migration & Recovery