What is the status of a mailbox when exporting the mailbox via the zmmailbox getRestURL "//?fmt=tgz" technique?

We have a client whose users have both multi GB mailboxes and fairly voluminous inbound email flows over a 24-hour period.

For a variety of reasons, we are migrating them to a new Zimbra server using the zmztozmig tool. The tool calls zmmailbox like so:

Code:
/opt/zimbra/bin/zmmailbox -z -m user@mydomain.com getRestURL "//?fmt=tgz" > /tmp/user.tgz
After the mailbox is exported, zmztozmig's conf file can be set to change the mailbox transport to forward all new inbound email to the new Zimbra server. (We'll change the MX record to point to the new Zimbra server after all the mailboxes are exported).

But while the mailbox is being gathered up into the tgz file, what happens to inbound mail destined for that mailbox?

Some of the mailboxes are 10GB to 30GB in size, so there will definitely be a lot of inbound email while the mailbox is being exported! We have been unsuccessful in finding any documentation to answer this, and we can't run the tool twice due to timing limitations in which the migration must be completed.

Best regard to all and TIA,
Mark