Ditto, almost always have the iPad in landscape mode. I type to much to fiddle around with portrait mode. Most of my emails themselves are fairly short. People generally send me longer things as attachments.
From time to time I'll do some reading in portrait mode, things like PDFs or eBooks. But mostly I'm replying or commenting, or doing something that requires the keyboard. And I type much more comfortably using all of my fingers in landscape.
Portrait mode for mail is something I hardly use, so it doesn't really bother me.