By default, the 1&1 webmail system will move deleted emails to the Trash folder where they can be viewed and restored if necessary. As a result, since deleting emails are simply transferred to the Trash folder, deleting emails from the Inbox does not free up space in your mailbox. To permanently remove these emails, they must also be deleted from the Trash folder.
As an alternative to this, you may set the configuration of the webmail system to permanently remove deleted emails. Using this option, deleted mails are not moved to the Trash folder, but deleted right away instead. Please note that by permanently removing deleted mails, you will not be able to recover any mail that has been accidentally deleted.