If you think a 'machine' is doing it, feel free to enable BoxTrapper which is in your Cpanel near the bottom. Then it'll auto send back a verification letter and if the machine doesn't return it, the letter is deleted.
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There is no Box Trapper on my Cpanel. I was told also that it didn't work well yet and is not on all Cpanels and may be removed in the future. Any other ideas?