Email Migration

Fix Zoho Mail to Microsoft 365 IMAP Migration Authentication Failures

Amos Gikunda Oct 26, 2026 868 views

Question details

The user needs to successfully migrate mailboxes from Zoho Mail to Microsoft 365, but the IMAP migration is failing due to authentication errors.

Fix Zoho Mail to Microsoft 365 IMAP Migration Authentication Failures

Product: Microsoft 365
Device & OS: not provided
Scenario: Migrating large user mailboxes from Zoho to Exchange Online using the IMAP migration tool in the Exchange Admin Center.
Observed behavior: The migration fails and returns an error stating that the IMAP server sent a BAD response to AUTHENTICATE with a ClientBug response code.

Before you start

Ensure you have administrator access to both the Zoho Mail Admin Console and the Microsoft 365 Exchange Admin Center, and verify that the target mailboxes are fully provisioned in Microsoft 365.

Solution 1Recommended

Enable Zoho IMAP Access and Generate an App-Specific Password

Resolve the authentication error by enabling IMAP on the source Zoho account and utilizing an App-Specific Password instead of the regular account password.

Zoho Mail enforces strict security policies that often block standard password authentication from automated tools like the Exchange Admin Center. To bypass the 'ClientBug' error, you must explicitly enable IMAP access in the Zoho mailbox and generate an App-Specific Password for the Microsoft 365 endpoint.

Enable IMAP in Zoho Mail

Log into the Zoho Mail interface as the user, navigate to Settings (gear icon), select 'Mail Accounts', click on the primary email address, and ensure the 'IMAP Access' checkbox is enabled.

Generate an App-Specific Password

Go to your Zoho Accounts security dashboard (accounts.zoho.com), select 'Security', click 'App Passwords', and click 'Generate New Password'. Label it 'Microsoft 365 Migration' and copy the generated password.

Recreate the Migration Endpoint

Log into the Microsoft 365 Exchange Admin Center, go to 'Migration' > 'Endpoints', and delete the failing endpoint. Create a new IMAP endpoint using 'imap.zoho.com', port 993, and SSL encryption.

Update the Migration Batch

Edit or recreate your migration batch CSV file. In the password column for the Zoho account, paste the newly generated Zoho App-Specific Password instead of the user's standard login password, then start the batch.

Enable Zoho IMAP Access and Generate an App-Specific Password

Test with a single mailbox: Before launching a full migration batch for all users, test the new endpoint and app password on a single, smaller mailbox to confirm that the AUTHENTICATE BAD error is resolved.

Solution 2

Perform a Manual PST Export and Import

Bypass the IMAP migration utility entirely by using a local desktop client to export Zoho emails and import them into Microsoft 365.

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Frequently Asked Questions

Why does Exchange Admin Center show a ClientBug error for Zoho?

This error typically occurs when Microsoft 365 attempts to authenticate with Zoho using a standard password instead of an App-Specific Password, or when IMAP access has not been manually enabled in the Zoho user's account settings.

What is the correct IMAP server and port for migrating Zoho Mail?

You should use `imap.zoho.com` (or your regional equivalent like `imap.zoho.eu`), port 993, and ensure SSL encryption is enabled in your Microsoft 365 migration endpoint configuration.

Can I migrate Zoho emails without using the Exchange Admin Center?

Yes. For a small number of mailboxes, you can sync the Zoho account to a local client like Outlook, export the data to a PST file, and then import that PST directly into the Microsoft 365 mailbox.

How do I generate an app-specific password in Zoho?

Log into your Zoho Accounts page at accounts.zoho.com, navigate to the 'Security' tab, select 'App Passwords', and generate a new password specifically for your Microsoft 365 IMAP migration.

Migration & Recovery