Yes the manual does say you can search the body of text but maybe that is only for certain mail, I've been trying and I can't get it to work, but then I just use the native mail app and it doesn't have that heading, just TO, FROM, SUBJECT and ALL Hopefully someone else will know more.
The IPad Forum is a community of members from all walks of life and from all over the world coming together to share our experiences and to help each other learn about this wonderful piece of technology the Ipad. There is a wealth of information already here in the many threads we have so doing some searches will answer most of the questions you have. If then you cannot find the answer you are seeking then by all means post your question. There are many friendly and informed members here only too willing to help you.
In theory it's supposed to work by selecting All,and for any account type. I've only ever seen it work on older MobileMe addresses and on accounts hosted on Exchange 2003 and 2007.