Unless something has changed with IOS 5 I'm afraid you can't delete old email addresses at least not on a stock iPad. Apparently they will disappear off your address list eventually but how long that takes is unknown. It's a good idea to input your email info into Contacts as this might help minimize the risk of one of these addresses being activated again. I'm hoping someone may know if there were changes with IOS5 as I'd love to be able to get rid of a few myself.
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.