In order to submit a new migration request to your emailbox your have to select
Mailboxes management -> Mailbox_list -> Import
before submiting a new request check the size your emailboxes, source and destination.
The whole process might take from a few minutes to a couple of hours (or more) in function of the size of the mailbox.
Please, do not submit more than one request for the same account before the first has finished.
Be sure that DNS propagation takes effect otherwise you might lose some emails.
Read the following page before submitting a request.
In case of gmail account, please have a look at the page