Step 1: Inventory the Current Email Environment

Start with a full account of what exists before touching anything. Pull every mailbox from your current platform’s admin console: user mailboxes, shared mailboxes, distribution groups, service accounts, and archived mailboxes. Document each one by size and account name. If it isn’t in the inventory, it will surface at cutover.

Step 2: Measure Total Data Volume

Export a detailed storage report from the source system. Measure each mailbox across inbox folders, sent items, drafts, deleted items, and personal archives. Include calendar data, contacts, and tasks. These consume space across platforms and get missed in estimates that only count inbox size. Add everything up to establish a baseline before the first batch runs.

Step 3: Identify Large and Problematic Mailboxes

Sort mailboxes by size and find the outliers. Mailboxes that exceed the destination platform’s limits need a plan before migration starts, not after. Set a threshold, 50GB is a reasonable starting point, and document the specific action for every mailbox that crosses it. Archiving, cleanup, and splitting are all options but each requires lead time.

Step 4: Assess Attachment and Media Volume

Attachments account for the largest share of mailbox storage in most environments. Run attachment analysis reports to understand how much data is stored as file attachments versus plain text. This affects decisions around deduplication and external link conversion and changes the final storage footprint significantly.

Step 5: Plan for Growth

Historical data reflects past usage, not future needs. Review email growth trends over the last 12 to 24 months, forecast forward one to three years, and add at least 20 to 30 percent buffer to the migration volume estimate. Running into capacity issues immediately after cutover is avoidable. It requires planning for it before the migration runs.

Step 6: Verify Destination Platform Quotas and Costs

Check the storage limits and pricing tiers of the target platform against the estimated volume. Identify gaps between current usage and destination capacity before the project is scoped. Document whether additional storage licenses or archiving solutions are needed and build those into the plan, not into a post-migration conversation.

Step 7: Document and Validate Before Moving Anything

Compile findings into a storage assessment report and get stakeholder sign-off before migration begins. This document drives capacity planning, licensing decisions, and migration phasing throughout the project. If stakeholders haven’t reviewed and approved it, the project doesn’t have a validated baseline. It has an assumption.

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