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The iPad keeps a separate, internal, 'recently used email address' list that is completely separate from, and not in any way related to, your contacts list. The Mac has the same list but, on the Mac, there's a way to edit/delete this list. Sadly and incredibly, on the iPad, no such facility exists - there is simply no way to delete this list.....
OK - the partial good news....this list is of 'limited length' and old email addresses eventually 'drop off the end' of the list...
Many Members have complained about this; maybe Apple will read iPad Forum and hopefully they'll eventually do something about it.
I am moving your thread to the help section so you can receive more answers