A successful mailbox login does not prove the mail migration is complete.
Test:
- external -> user;
- user -> external;
- internal -> internal;
- application/device relay;
- aliases/groups/shared mailboxes;
- replies;
- attachments;
- SPF/DKIM/DMARC;
- gateway and connector paths;
- quarantine/filtering;
- message tracing/logging;
- mobile/client access where relevant.
Keep the old platform observable during the stabilization period so forgotten senders or cached routes are visible.