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.
CloudM Migrate, by default, will modify the date in such cases so that the emails can be migrated, but this setting can be disabled.
See page (4) > Advanced settings > Email Options > Modify Invalid Messages. It can be set to FALSE if you wish for this not to happen, in which case, it will not modify the messages and fail them as having invalid DATE: headers.
Be certain if you wish to disable this setting. It also affects other invalid aspects of messages such as being oversized or having bad attachments. It is enabled by default to ensure that all items migrate despite these invalid elements.