Did Sauron know about the Istari?

[Annunfuiniel]

Was he ever aware that, in Gandalf and Saruman, he was dealing with Maia?

Hmmmm... I suspect that Sauron wasn't really familiar with the true nature of the wizards. Or at least he didn't share his information with his servants, ie. the Lord of Nazgûl. That the Sorcerer didn't know what he was against with at the Gate of Gondor is quite clear from his words addressed to Gandalf. "Old fool!....Die now and curse in vain!" (sorry, I'm not sure of the exact quote) isn't something you'd choose to say when facing a maia, don't ya think?