Your e-mail does not move with your domain when performing an internal domain transfer. Instead, the e-mail accounts associated with the domain that was transferred to another package are reassigned to a new domain, since the domain they were created with is no longer avialble.
The e-mail accounts will be re-assigned to the domain @mailboxbackup.info, so that an e-mail address originally named firstname.lastname@example.org will be reassigned to email@example.com.
You will need to re-create the firstname.lastname@example.org e-mail address in the package that the domain was transferred to, and then import the mail from email@example.com to the re-created firstname.lastname@example.org in the webmail interface using the steps below.
By clicking on the Inbox at the top-left of the screen, you can now confirm that the e-mails have been copied from the @mailboxbackup.info account to the Inbox for this e-mail account.