To delete ALL messages in ANY folder :
Highlight one of them, anyone > press CTRL key and HOLD > press A key > all
the messages are now ' shaded ' > click Delete
To delete Some messages in Any folder :
Start at the top of the list, highlight it > scroll to the last of the list,
press Shift key and HOLD > click the last on the list, all messages in
between are now ' shaded ' > click Delete
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