Did Sauron know about the Istari?

[The Saucepan Man]

... until Saruman became his servant,of course

I'm not sure that Saruman was ever Sauron's servant. Sauron may have thought of him as such, but I'm sure Saruman had other ideas ...

I would agree with Legolas and Estanesse that the existence of the Istari, as an order of Wizards, would have been known to Sauron. So the question is when, if ever, did he become aware that they were Maia.

... I don’t think he knew that the Istari were Maia until they attacked him at Dol Guldur

Is that right? Did he become aware of this during the attack on Dol Guldur? Was he ever aware that, in Gandalf and Saruman, he was dealing with Maia?

The point is of particular interest with regard to Saruman. If he considered Saruman to be a mere wizard, rather than a Maiar, then he would no doubt have been fairly confident of his ability to dominate him. But, if he had been aware that he was a Maiar, he would surely have done everything in his power to prevent Saruman's orcs getting Merry and Pippin (who both he and Saruman assumed had the Ring) back to Isengard. After all, a human wizard with the Ring would be fairly easy to deal with. But a Maiar with the Ring would be another matter ...