Most email migration problems come from a handful of causes: an incomplete inventory, weak coexistence design, underestimated data volume, forgotten applications that send mail, and poor communication with users. Each shows up as a recognizable symptom during or after cutover, such as missing mail, broken calendars, failed batches or a flood of helpdesk calls, and each can be prevented with the right step in the plan.
This article is a troubleshooting companion to our email migration plan and checklist. It covers the Office 365 and Microsoft 365 migration issues we see most, what causes them and how to avoid them. RockTeam handles these projects through our email migration services.
| Symptom | Likely cause | Prevention or fix |
|---|---|---|
| Mail stops arriving for some users | MX, connectors or routing changed before mailboxes moved | Plan coexistence routing and switch MX only after most users have moved |
| Users cannot see each other’s free/busy | Coexistence not configured between source and target | Set up hybrid or cross-platform free/busy before the first wave |
| Delegates lose access to a manager’s mailbox | Manager and delegate moved in different batches | Move executives, assistants and shared mailbox users together |
| Migration batches fail or stall | Unhealthy source servers, throttling or oversized items | Fix source health, raise message size limits and pilot to measure throughput |
| Cutover window runs far past plan | Data volume underestimated | Pre-stage older mail and move only recent mail at cutover |
| Printers, scanners or applications stop sending mail | SMTP relay still points to the old server | Inventory every mail-sending device and app and reconfigure relay |
| Users report missing old mail | Archives or PST files left out of scope, or retention policies acting on moved items | Include archives and PSTs, and pause retention policies during migration |
| Outlook or phones keep prompting for passwords | Autodiscover or authentication changes not planned | Update Autodiscover, plan modern authentication and give users clear steps |
| Helpdesk is overwhelmed after cutover | Users not told what will change or when | Communicate in advance, provide quick guides and staff a support surge |
Source specific guidance: Exchange to Office 365 migration, Google Workspace to Microsoft 365, tenant to tenant migration and Lotus Notes to Office 365 migration.
Want a migration that avoids these problems?
RockTeam’s migration architects design coexistence, pilots and wave plans that keep mail flowing and users productive, from 100 to well over 100,000 users. Start with a no cost assessment.
See Email Migration ServicesMissing mail from routing changes, broken free/busy, delegates losing access, failed or slow batches, applications that can no longer send mail, missing archives and overwhelmed helpdesks after cutover.
Common causes are unhealthy source servers, throttling, oversized items above the message size limit, mailboxes on hold, and missing licenses or permissions in the target.
Include archives and PST files in scope, pause retention policies during migration, validate item counts for each mailbox and keep migration reports until users sign off.
Permissions often do not work across systems during coexistence, so managers, delegates and shared mailbox users should be moved in the same batch.
Design coexistence first, run a pilot with demanding users, migrate in phases, pre-stage data and communicate clearly with every wave.
