Hi Ian,
About your first question, I want to know where did you run the message trace, within the old tenant(xxxx.gov.uk) or the new tenant(company B's xxxxx.co.uk)? If you run the message trace within the new tenant, please check the report in the new tenant’s
Exchange admin center. Also, I want to check whether the old tenant is still in use or has been expired? If you ran the message trace within the old tenant and it has been expired for 30 days, the content will be removed.
Second, according to your description, it seems that you download the report via the link in the notification email. You might run the message in the old tenant but send the notification email of the message trace result to your account, which has migrated
to another tenant( migrate to company B's xxxxx.co.uk from xxxx.gov.uk ), so you don’t have the access to download the report. Another scenario is that the recipient of the result doesn’t have the admin access. Please check the permission of the recipient’s
account.
I suggest you run the message again and then download the report using an admin account. Please make sure you run and download the message report within the same tenant.
If there is something I misunderstood you, please let me know the recipient’s domain, capture a screenshot of the permission, and provide a screenshot of the full error message.
Best regards,
Rick