The iPad's Mail app keeps a list of 'frequently used recipients'. Mail on the Macintosh does the same but, on that computer, there is the facility to clear that list. Unfortunately no such facility exists on the iPad.
Several members have complained about this and there is no known way to purge the list of names - particularly annoying if it contains mistyped names that have been accidentally entered and are now continually 'suggested' as you start to type a new email.
The only good news is, I'm afraid, that the list is not infinite and those names eventually 'fall off the end' of the list.
Tim