"Rude" has a scale. If they're being blunt because they're disappointed or frustrated, it may be interpreted as rude. You'll want to satisfy them.
If they're being rude because they're an idiot or maybe they represent an outside group of your market, a good marketing trick is to turn their rudeness into email content. A common enemy if you will. You win by building your loyal following. You'll get many people replying back encouraging you.
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