I'm using this software as provided by the hosting company 1&1. I can view messages in the Spam folder and click on them to say they are Not Spam, in which case they go to the Inbox. However, a subsequent message from the same e-mail address is still treated as spam.

How can I arrange that a given sender's e-mails (from the same e-mail address) are never spam?
Remember, I am just using the software as seen & implemented in 1&1 Webmail.

Thanks in advance.