In the mail app. How to copy and paste?

In the Snow Leopard I was able to start a new message and go to other messages in Mail the new message window went behind the mail app but now in Mounrtain Lion when the new mail message window is on top I can not go to the other mail messages. The new mail windows locks the mail app until I press send or cancel.

I like to be able to go to the other messages in my in and send box to copy and paste rlevant data to the new message.

I went through the prefferences but no avail.

Is there a setting to change or is this the way it is?

Thanks.