I am transferring my emails from old mac to new mac, using mbox file method.

After import, some emails' receive time shows the import time, not the real received time - which shows correct on old mac outlook.

Half folder on new mac has this problem, and not all emails in this folder has this problem, e.g some shows wrong receive time while others still correct.

Can anyone suggest what's going wrong and how to fix it?  Thanks in advance.