There's more to it than her just being single, or wanting to stir things up, or wanting to tell WH that she's available (whether that's true or not).
She's jealous of what you have and she wants to spoil your lovely post-birth time with your baby. (This may be because WH has said to her that the baby has made him reconsider his priorities, or it may just be because the emotions are all hers).
Don't do anything other than reply to her to say, please do not contact me again. Don't show the email to WH, just in case he does not know about the divorce. I'd just say, OW has emailed me and then fall silent.
If he asks what she said, say, what do you think she said? He's been, at best, disingenuous so far, so it's your chance to find out what he thinks is going on.
But the flip side of this is that it might not be his doing, and more down to her trying to elbow in one more time. But I'd probably be getting someone else in to help, rather than him for the next few days, just to give him the message that any kind of out of order-ness will push him away for good.