When moving a domain out of your current package, any e-mail addresses that were still actively using the domain will be renamed. For example, if the domain 1and1help.co.uk was moved to another package, the active e-mail address firstname.lastname@example.org would be renamed to email@example.com. This way your mailbox remains, but the e-mail address is renamed.
After the domain has transferred to the other package, you may recreate the e-mail address in that package using the exact same name as before. When re-createing the e-mail address after the domain transfer, the inbox will be empty. You should log in to 1&1 WebMail for that account and follow the steps below to transfer the e-mails from the old e-mail address to the newly created, empty mailbox.