I just bought a MacBook Pro and I find the same problem. When composing an email the program indents the first line.
This is probably the wrong place to bring this up, but I have been forced to be a WIndows user for many years in my work. I have had a lot of my Mac owning buddies tell me I should get a Mac now I am retired. They say it is far superior to Windows. Well, I'm not seeing it so far. I know all new things take getting used to, but some of the stuff is just weird. Such as the "Save As" command disappearing from Mountain Lion. Apparently Apple is trying to take everyone down the IOS road. Well, I have an iPad and I love it. But I bought the MacBook so I can get some serious work done. Only to find Apple playing social engineer.
So I'd be happy if the email client didn't indent stuff for me, thanks all the same.