Good question! If you don't get an answer here, please put in a support ticket.
Looks like it shows for Email to Case it will be queued and then processed. Please open the case with BMC support, they can open case with Salesforce to help you read this as well
BMC opened a ticket with Salesforce for me and got an answer on this question. From Salesforce support:
"...incoming mail during an instance migration...will queue for up to 24 hours worth of down time, and will be received by the org once maintenance is over."