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Google AdWords vouchers are available from several companies online as added value inclusions in a purchase i.e. "Buy this and receive £x of Google AdWords vouchers".
I've personally dealt with Google in setting such an offer up and know how stringent they are on how the process will work - so to receive a voucher out of the blue seems strange.
Have you bought something recently that the voucher coude have been included with as a special offer? Is there any branding on the email to see who sent it i.e. was it from a specific company or was it straight from Google? Or was it from someone you've never heard of?