Sometimes, the migration date will appear on some emails after the migration.
This happens when mail from the source has an invalid DATE: header - "invalid" meaning not compliant with RFC822. We modify these messages by default to ensure they are migrated.
If you do not want the messages to be modified,
- On the Config Settings screen, select Advanced Settings > Email > Modify Invalid Messages
- Set to Disabled.
This will cause the message to not be modified and it will fail the migration (as it will have an invalid DATE: header).