Inventory data and dependencies
List users, role addresses, aliases, delegates, folders and stored sizes. Include website forms, devices, calendars, contacts and applications that send email. Record retention requirements and who approves the move.
Choose an export or backup method suitable for the existing provider, then verify that a sample can be opened. An export file that nobody has checked is weak recovery evidence.
Separate mail from other information
Do not assume a mail-transfer tool moves everything. Microsoft’s IMAP migration documentation states that contacts, calendars and tasks are not migrated by that method. Plan separate transfers and tests for information outside email folders.
Record which method moves each data category and which features will not transfer automatically. Pay particular attention to delegates, rules and shared resources rather than discovering their absence after everyone has switched.
Pilot before changing delivery
Use a small representative test group, including a large mailbox and a role address if applicable. Compare folders, sample messages, dates and attachments. Check permission boundaries and sending identities.
Agree acceptance criteria in advance. For example: selected old messages open, expected folders are present, inbound and outbound tests succeed, and an unauthorized account cannot access the finance mailbox.
Plan the cutover and rollback
Assign an owner, a staffed change window and a communication channel that does not depend on the mail being moved. Save the current DNS values and describe when you would pause or revert.
Restoring old DNS records alone does not reconcile messages already delivered to the new provider. Include a plan for that split history. Keep overlap long enough for verification and the agreed retention requirements; do not delete the old service just because the new login works.
Sign off the evidence
Maintain an acceptance table with check, owner, result and corrective action.
- Incoming mail and replies tested from outside the organization.
- Authentication checked for employee mail and application senders.
- Representative old messages and attachments opened.
- Contacts and calendars checked using their separate migration method.
- Users informed, recovery available and old-service retirement approved.
Sources
- What you need to know about IMAP migration — Microsoft Learn. Reviewed .