In Pine or Alpine, you can quickly select and delete all messages using a few simple key commands. First, press ; to start a selection command, then press a to select all messages. After doing this, a capital X will appear at the beginning of each message line, indicating that these messages are marked for an action. Marking a message does not perform the action immediately; it simply flags the message so that a command can be applied to it. To mark the messages for deletion, press a to open the Apply Command menu, then press d to choose the Delete action. At this point, all messages have been marked for deletion, but the deletion will only actually occur when you quit the program in the usual way by pressing q, or you can delete the messages immediately by pressing x to execute the expunge command, which removes all messages marked with D right away.
Summary of key points:
; → start marking
a → select all messages → marks them with X at the beginning of each line
a → apply command
d → mark messages for deletion
q → exit program → messages marked for deletion are actually deleted
x → expunge → immediately deletes all messages marked with X