I Have More than One E-Mail Address - Does this Mean that I Have More than One Mailbox?
Users can have two or more e-mail addresses simultaneously, e.g. firstname.lastname@example.org and email@example.com. This means that there is only one mailbox, reachable under different e-mail addresses. Also your e-mail-quota (storage size) remains the same.
You can set those e-mail addresses in TUMonline.