We are planning to move users from Intermedia to Microsoft 365, but some accounts have unusually large mailboxes and calendars. One calendar contains about 17,000 items and 6.8 GB, while another has more than 65,000 items and is around 114 GB. Some mailboxes are also larger than 200 GB.
Microsoft 365 has mailbox and archive limits, so I'm trying to determine whether a migration tool can automatically route older mail into the online archive, remove or skip years of obsolete calendar entries, and keep only recent content in the primary mailbox. Or do we need to clean everything up manually before starting?
What limits and migration approach should we plan around? The hosting provider has offered to perform the migration, but I want to understand exactly how they would handle these oversized accounts before agreeing to the process.
3 Answers
The safest approach is to plan the destination layout before migrating. Enable Exchange Online archiving, then move older mail directly into the archive and keep only the most recent year or two in the primary mailbox. The exact limits depend on the Microsoft 365 license, but an Exchange Online Plan 2-type license is generally needed for the larger archive capacity, along with auto-expanding archive if the archive may exceed 100 GB.
Make sure the migration tool can specify date ranges and the target mailbox versus archive. Otherwise, a staged migration can see archived items as missing from the primary mailbox, copy them again, and create duplication or repeated archive processing.
A staged migration can work with tools that support archive targeting. A typical sequence is: enable archives for the users, migrate content older than a selected cutoff directly to the archive, migrate newer content to the primary mailbox, and run a final pass for anything that changed before cutover. Confirm the licensing requirements first, since the archive migration may require additional or different licenses during the process.
Do not assume the migration tool will automatically clean up old calendar entries. Calendar data should be reviewed and reduced separately, especially with tens of thousands of items. Exporting selected date ranges to PST can be a fallback if the tool cannot handle the source data cleanly.
This staged approach has worked for large moves, but test it with a representative mailbox first. Verify that recurring meetings, delegates, shared calendars, and archived items behave correctly before applying it broadly.
The oversized mailboxes can usually be managed with the destination’s archive policies, but the calendar counts are a separate risk. A calendar with 65,000 or more entries can slow down or disrupt migration jobs, so inventory and clean up obsolete appointments before the move. Decide whether old calendar data must be retained, exported for records, or deleted according to the organization’s retention requirements.
Before starting, get the migration provider to document the applicable primary-mailbox and archive limits, how items over those limits are handled, whether calendar date filtering is supported, and what happens when an item cannot be migrated. Run a pilot first rather than discovering these problems during cutover.
The provider’s offer may be useful, but ask them specifically whether their process supports separate date ranges and direct migration into the online archive. Also ask how they prevent archived data from being copied back during the final synchronization.