Migration
Enterprise Mail Migration
Migrating mailboxes from on-premises mail servers to Exchange Online, with coexistence and a planned cutover.
Scope
- Migration planning from MDaemon / Zimbra / IMAP sources to Exchange Online
- Mail flow coexistence during the migration window
- DNS and routing changes for cutover
- Mailbox and distribution list migration
Approach
- Inventoried mailboxes, distribution lists, and mail flow rules on the source server first.
- Set up coexistence so mail kept flowing correctly to both systems during migration.
- Migrated mailboxes in batches, verifying folder structure and permissions after each batch.
- Planned and executed DNS cutover with a rollback path if issues appeared.
Context
Migrating a company’s mail system is one of the least forgiving changes in IT — if it goes wrong, everyone notices immediately. This project covered migrations from legacy mail platforms (including MDaemon, Zimbra, and generic IMAP sources) to Exchange Online, with an emphasis on keeping mail flowing throughout.
What this involved
Before any mailbox moved, the source environment was inventoried: mailbox count and size, distribution lists, shared mailboxes, mail flow/transport rules, and anything mail-routing-dependent (ticketing systems, CRM integrations, automated notifications). Coexistence was configured so that mail routed correctly between the legacy system and Exchange Online for the duration of the migration — critical for organizations that can’t cut over all at once.
Mailboxes were migrated in batches using the appropriate tooling for the source platform, with folder structure, permissions, and delegate access verified after each batch rather than assumed.
Approach
Cutover (the DNS and MX record change that sends new mail to Exchange Online) was planned as a discrete, reversible step — scheduled for low-impact hours, with the previous DNS configuration documented and ready to restore if mail flow issues appeared after the switch.