Migrating from on premises Exchange to Microsoft 365 does not end the moment the last mailbox finishes moving. The old Exchange environment usually needs a deliberate cleanup pass afterward, and skipping it is one of the more common reasons organizations end up running an unnecessary on premises server for years longer than they meant to.
Decommissioning the old Exchange server properly
Even after every mailbox has migrated, Exchange servers are frequently kept running because they are still handling hybrid mail flow, still hold the Exchange schema extensions in Active Directory, or because nobody wants to be the one who turns it off. Microsoft's own decommissioning guidance for hybrid environments should be followed in order, since removing an Exchange server incorrectly can break mail flow or leave orphaned Active Directory attributes that cause problems much later.
Cleaning up mailbox database files
Once mailboxes are confirmed migrated and verified, the Exchange database and log files on the old server can be reclaimed, which is often a meaningful amount of disk space on a server that has been running for years. Confirm backups of the final state exist before removing anything, since this step is not reversible.
Cleaning up Active Directory and DNS
Hybrid Exchange setups create a number of DNS records and Active Directory objects that outlive their usefulness once the migration is fully complete and the last Exchange server is gone: autodiscover records pointing to the old environment, legacy mail routing connectors, and Exchange specific AD groups. Leaving these in place does not usually break anything immediately, but it does leave stale configuration that makes troubleshooting harder for whoever looks at the environment next.
Confirming hybrid mail flow settings still make sense
If your organization kept any hybrid connectors for mail flow reasons, revisit whether they are still needed once Exchange itself is gone, since a connector pointing at a server that no longer exists is a common cause of mysterious mail delivery delays.
If your Exchange to Microsoft 365 migration finished a while ago and the old environment was never properly cleaned up, that is a very fixable and often overdue project. Our cloud migration team can help close it out properly. Get in touch if that describes your situation.