Onpremise Exchange Server to M365 Migration issues & Solutions 

1. Pre-Migration Issues

• Incomplete Inventory: Missing information about mailboxes, distribution lists, or shared mailboxes.

  • • Use tools like the Microsoft Assessment and Planning (MAP) toolkit or Exchange Deployment Assistant to discover all mailboxes, distribution lists, and shared resources.

    • Prepare a detailed migration plan with all dependencies.

• Network Bandwidth Constraints: Large data transfers can cause slow migrations or failures. 

  • • Perform bandwidth tests and schedule migration batches during non-peak hours and weekends.

    • Used staged migration to spread the workload.

• Mailbox Size Limits:  Office 365 imposes size limits (typically 100GB for E3/E5 mailboxes). 

• Archive large mailboxes using In-Place Archiving or reduce mailbox size by cleaning up old emails.

• Used PST export tools for excess data.

• DNS Configuration Issues: Incorrect Autodiscover and MX records disrupted the connectivity. 

  • It is better to verify the DNS entries (Autodiscover, MX records) in advance and plan DNS cutover carefully.

  • Use Microsoft’s Remote Connectivity Analyzer to validate DNS and Autodiscover.

2. Identity and Authentication Challenges

• Active Directory (AD) Sync Errors: Issues with Azure AD Connect, such as attribute mismatches or sync failures.

  1. • Ensure proper attribute mapping and install the latest version of Azure AD Connect.

     • Check for duplicate attributes like email addresses or UPNs and resolve conflicts.

• Password Sync or Federation Problems: Inconsistent or broken password sync between on-prem AD and Azure AD.

  1. • Run diagnostics on Azure AD Connect and ensure proper service account permissions.

     • If using Federation Services (ADFS), ensure SSL certificates are valid and trusts are configured correctly.

• Multi-Factor Authentication (MFA) Disruptions: Users may face login issues if MFA isn’t properly configured before migration.

  1. • Notify users in advance about MFA changes and enforce MFA rollout in stages.

     • Use conditional access policies to gradually implement MFA.

3. Mail Flow and DNS Issues

• MX Record Propagation Delay: Delays in updating DNS records can result in lost or delayed emails.

  • • Reduce the TTL (Time to Live) value on DNS records before migration to speed up propagation.

    • Monitor MX records using online DNS tools to ensure proper cutover.

• Email Relay Problems: On-prem devices or applications using Exchange as a relay may stop functioning.

  • • Configure SMTP relay connectors in Exchange Online for on-prem devices and applications.

    • Verify authentication and IP allowlists for relaying.

• Spam Filtering Issues: Misconfigured spam policies between Exchange Online and on-prem systems.

  • • Configure Exchange Online Protection (EOP) and allow lists based on current spam policies.

    • Monitor mail flow logs to adjust spam rules.

4. Data Migration Issues

• Corrupt Mailbox Items: Damaged or corrupt emails can interrupt the migration process.

  1. • Use Exchange Mailbox Repair tools or PowerShell commands to detect and fix corrupt items.

     • Exclude corrupt items during migration using -BadItemLimit parameters.