This is Mailbox Sweeper right after startup. In the box on the left, you see the mailboxes
specified in profile.dat. You can select a mailbox, and press the button labeled
"Check mails" to log into the mailbox to check for new emails.
To edit profile.dat, either use a text-editor to edit it directly, or go to File->Edit profile to edit it from within Mailbox Sweeper.
This is how it looks like after you checked into a mailbox. In the box in the middle the Subject lines of the messages and the respective senders are displayed. Note the two buttons that are now visible labeled "Delete selected mails" and "Delete all mails in mailbox".
You can now either delete single/multiple messages by selecting them and then pressing the button labeled "Delete selected mails", or delete all messages by clicking on "Delete all mails in mailbox". Note that this button will delete *all* messages in the box, including any mails that are not shown in the message-box.
You can specify how many emails should be displayed by editing the appropiate value in profile.dat.