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On Windows, you'd use the control key to select multiple items, but on OSX you use the command key.
That said, command click does nothing inside webmail on OSX and while I can use the control key; coupled with a mouse click to select each item, it also fires the right click menu action which makes selecting multiple emails annoying as heck!
Given the issue outlined with the delete key not working on OSX for deleting items, previously mentioned here:
...there is work needed on the UI to handle OSX/Mac users (we do exist)