Inboxes of e-mail clients are the folders in which incoming e-mails are stored. Every incoming e-mail goes first to the inbox, which is called inbox in English.
All incoming mail is stored in the inbox before it is filtered in mail filters and sorted into individual folders. Spam mails are filtered out in spam filters and automatically moved to the junk folder. Depending on the e-mail software, the incoming but not yet accessed e-mails are marked, for example in Thunderbird by a bold font size. In order to handle the number of incoming e-mails sensibly and clearly, there are subfolders in the inbox into which the e-mails are sorted according to topic or user.
The outbox is called outbox in English.