In Caching mode, you can delete items, retract items, and empty the Trash the same way you would in the Online mode.
Click the item in your Mailbox.
Click Edit, then click Delete.
If you're deleting an outgoing item, click one of the available options.
To free up disk space, click Edit, then click Empty Trash.
This permanently removes all deleted items from your Caching Mailbox.
Changes in your Caching and Online Mailboxes will automatically synchronize.