Move IMAP mailboxes to Microsoft 365: what moves and what does not

Define the content inventory

IMAP migration concerns inbox and other mail folders. Microsoft says it does not migrate contacts, calendar entries or tasks. Inventory those separately, along with local-only archives and shared-mailbox needs. The published tool limits are up to 500,000 items per mailbox and 35 MB per message; identify exceptions before promising complete transfer.

• Record mailbox counts and approximate sizes without exporting message contents.
• Check whether retention or archive policies affect verification.
• Create and license the intended destination mailboxes before migrating.

Access cannot be solved by emailing a password list

Microsoft's migration procedure requires authorised access to source mailboxes. That is an administrator-controlled requirement, not an invitation to send passwords or migration CSVs to an enquiry or agent. Agree a secure owner-held execution route and data boundaries before accepting work; if unavailable, migration cannot simply proceed.

• Do not attach a mailbox export, password CSV or staff directory initially.
• Name the authorised tenant and source-mail administrator.
• Agree user communication and any password impact before changes.

Old mail and new routing are different checks

Verify copied mail folders and any exceptions before changing MX routing. After the approved cutover, verify new mail reaches the destination and that required sending paths work. Microsoft places routing verification before stopping synchronisation. A successful login to one new mailbox does not prove that old messages or every address are correct.

• Retain the source until the agreed verification and recovery boundary.
• Account for aliases and authorised senders in the DNS/authentication plan.

Scope and evidence

A credible proposal lists included mailboxes, excluded data types, size exceptions, verification criteria and who performs the authorised account changes. Send providers and approximate counts initially. Exact private inventories and access remain gated; no zero-loss or zero-downtime guarantee follows from this guide.