I have looked around a bit myself and have been driving myself crazy. It appears that the Mac email app maintains a 'recent recipients' file, but that there is the option to delete it.
As twerppoet has discovered, there doesn't seem to be such a facility on the iPad to delete this file. But, surely, the size of the recent recipients file must be limited? Won't these spurious email addresses eventually 'drop off the end' of the file?
The search goes on