In Mail.app 6.0 with 10.8 Mountain Lion, Apple has chosen to make it impossible to drag-select text in a message that has been sent or received. In previous versions, you could do as you can with every other single program that handles user-created text anywhere in the operating system (!!!) and put your cursor anywhere in the message, drag to highlight the text you want, and then, voilà, there'd you have a discrete selection of text that you could, say, copy.
Now, in Mail.app 6.0, when you try this in the body of a sent or received message, you instead get a little mail icon. Mail.app assumes you want to drag the whole message somewhere else! This is highly counterintuitive program behavior.
It is impossible to drag-select text in a sent or received mail message, as far as I can tell with the settings Mail.app currently has. The only approximation is to double-click on a word (which selects it; triple-click will select a sentence), then hold the shift key, then click on the end of the text you want to select, then copy or whatever as you like. But even with this method it is impossible to select just a part of a word.
How can I revert Mail.app 6.0 to the proper drag-select behavior that works absolutely everywhere else on the system? The way Mail.app 6.0 behaves for this is counter to my 24 years of history with the various Mac OSes.
UPDATE: It seems to have resolved itself. I am not sure what did it but now it behaves as it should.