Exchange Online Shared Mailbox Resolver & Migration Planner Pre-Processor

This script pre-processes Microsoft 365 / Exchange Online mailbox export CSVs to ensure 100% resolution when scanning Shared Mailboxes in the Google Workspace Migration Planner.

Prerequisites & Requirements

Before running this script in a environment, ensure you have:

1. System Requirements & Module

• PowerShell 7+ (Recommended) or PowerShell 5.1 on Windows/macOS/Linux.
• ExchangeOnlineManagement Module (v3.0.0 or later):

  Install-Module -Name ExchangeOnlineManagement -AllowClobber -Force

2. Required M365 Permissions

The admin account running the script must have at least one of the following roles in Exchange Online:

• Global Reader
• Exchange Administrator
• View-Only Recipients

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Step-by-Step Guide

Step 1: Prepare the CSV

Obtain the list of shared mailboxes.

• The CSV can contain any standard header name for emails (Email Id, Email, PrimarySmtpAddress, UserPrincipalName, Identity, or EmailAddress).
• Example input.csv:

  Email Id
  sales@company.com
  support-alias@company.com
  info-shared@tenant.onmicrosoft.com

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Step 2: Execute the Script

1. Open PowerShell and navigate to or run UPN_SHaredMailboxes.ps1:

   .\UPN_SHaredMailboxes.ps1

2. Authenticate with Exchange Online:

   • If not already connected, an interactive M365 sign-in prompt will open. Log in using an account with appropriate Exchange Online read permissions.

3. Provide Input File Path:

   • When prompted:

     Enter the path to your input CSV file (e.g., C:\Migration\input.csv):
     

   • Drag and drop the CSV file into the terminal window or type the full file path.

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Step 3: Script Execution & In-Memory Processing

The script will perform the following steps automatically:

1. Pre-fetch: Downloads tenant mailbox data in a single batch.
2. Hashtable Mapping: Maps every primary email, UPN, and alias to its respective Exchange Online mailbox object.
3. Instant Lookup: Resolves all input rows in seconds without incurring Graph API throttling.

Sample Output:

Connecting to Exchange Online...
Successfully loaded 1808 entries from CSV.
Using column 'Email Id' for Exchange Online lookup...
Processing... Please wait.

Pre-fetching Exchange Online mailboxes into memory...
Loaded 4250 email address/alias mappings from Exchange Online.

============================================================
                    PROCESSING COMPLETE                     
============================================================
 Total Input Rows           : 1808
 Successfully Resolved      : 1808
 Shared Mailboxes Found     : 1750
 User Mailboxes / Other     : 58
 Not Found / Skipped        : 0
============================================================

OUTPUT FILES GENERATED:
 1. Migration Planner CSV   : C:\Migration\MigrationPlanner_SharedMailboxes_UPN.csv
 2. Full Audit Report       : C:\Migration\ExchangeOnline_Full_Audit_Report.csv

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Step 4: Upload Clean CSV to Google Workspace Migration Planner

1. Open the Google Workspace Migration Planner tool.
2. Navigate to the Shared Mailboxes scan/discovery section.
3. Upload MigrationPlanner_SharedMailboxes_UPN.csv generated by the script.
4. Result: The Migration Planner will resolve 100% of valid shared mailboxes without dropping entries or failing on alias mismatches.

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Output Files Reference

File Name	Description	Purpose
MigrationPlanner_SharedMailboxes_UPN.csv	Clean CSV containing single column Email Id populated only with exact Entra ID UPNs of verified SharedMailbox accounts.	Feed directly into Google Workspace Migration Planner.
ExchangeOnline_Full_Audit_Report.csv	Comprehensive diagnostic report containing Input Email, UserPrincipalName, PrimarySmtpAddress, RecipientType, and Status.	Use for review, identifying non-shared mailboxes (e.g. UserMailbox), or investigating missing accounts.

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🛠️ Troubleshooting & Notes

• Module not found: If Connect-ExchangeOnline fails, run Install-Module ExchangeOnlineManagement -Force.
• Not Found / Skipped Entries: Check ExchangeOnline_Full_Audit_Report.csv for any rows marked NOT_FOUND to verify if accounts were deleted or misspelled in the CSV.
• UserMailbox in Shared Scan: If regular user accounts are included, they will be listed as UserMailbox in the audit report and automatically excluded from the Migration Planner output CSV.