I would think this is suspect ... or you've simply not realised it was an offer included in something you've bought recently.
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?