Affirmative.

During migration, and especially when you're moving many mailboxes, and can only move some at a time, it's possible for us to redirect all mailboxes you haven't yet created on the 01.com servers to your old server. That way, once you've moved your MX record to the 01.com servers, mail for any mailboxes you haven't yet created at 01.com will be forwarded to your old server, and delivered into your respective, old mailboxes.

After you sign-up for service, open up a ticket with our help desk with the title "please redirect mail with unknown mailboxes to our old server," and put the details about your old server in the body of the help desk ticket, such as it's IP and host name.