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An old mentor of mine read some of my early attempts at copy and told me "Knocking copy never works" so I have avoided it ever since.
Knocking copy is simply where you show yourself in a far better light than the competition.
However, many of your prospects may be unaware of the competition and may decide to seek them out to see if you are telling the truth.
It can also appear that you are struggling to find strong benefits of your own and your knocking copy makes you look bitter and twisted.
Recently though I've seen adverts for Lidl and other stores that price compare their brands against the brand leaders. How they get away with copying the packaging of the major brands heaven knows.
The fact that Lidl has grabbed market share from Tescos, Morrisons etc suggests that this approach has worked.
On balance, I will probably keep away from knocking copy but when you are faced with a "Goliath" competition it might just be worth a try.