We're planning to move users from Intermedia to Microsoft 365, but some mailboxes and calendars are far beyond the usual Exchange Online limits. One calendar has about 17,172 items and 6.82 GB, another has roughly 65,167 items and 114 GB, and some mailboxes exceed 200 GB. Intermedia apparently didn't enforce comparable limits. Can a migration tool archive older content during the migration, or do we need to clean everything up manually first? What Microsoft 365 limits, licenses, and migration steps should we plan around?
4 Answers
Don’t assume the provider’s migration service will solve the limits automatically. Get a written plan covering primary mailbox limits, archive capacity, auto-expanding archives, calendar handling, date cutoffs, duplicate prevention, and the final synchronization. Test the process with a few representative users first, especially those with mailboxes over 200 GB and unusually large calendars.
The usual approach is to enable Exchange Online archives first, then migrate older content directly into the archive and keep only the most recent year or two in the primary mailbox. A migration tool may be able to use date ranges and specify the destination, but verify this carefully. Otherwise, the tool can see archived content as missing from the primary mailbox and copy it again, causing duplication or repeated migration passes. You’ll likely need an Exchange Online Plan 2-level license, and auto-expanding archiving may be necessary for very large archives.
The mailbox sizes can generally be handled with Exchange Online archiving and retention policies, but the oversized calendars deserve separate planning. Tens of thousands of calendar items can slow down or disrupt migration jobs. Clean out obsolete calendar entries beforehand if possible, or at least confirm with the migration provider what item counts and migration times they support. Calendar cleanup may need to be a manual process rather than something performed automatically during migration.
A staged migration can work with tools such as BitTitan or AvePoint. A common sequence is: enable archives for the users, migrate content older than a chosen cutoff directly to the archive, migrate newer content to the primary mailbox, and then run a final pass at cutover for anything that changed. Check the licensing requirements because the archive workflow may require additional licenses during the project. Make sure the tool supports this destination-aware process before starting.
BitTitan is what the current provider uses, so I’ll need to confirm that it can send older items directly to the archive instead of treating them as missing from the primary mailbox.
Intermedia offered to perform the migration, so I’m going to confirm exactly how they handle date ranges, archives, and oversized mailboxes before proceeding.