Just got this email reply:
Thank you for your ticket.
We apologise that due to unprecedented demand for The Witcher 3 from Green Man Gaming, the launch and dispatch of keys to you today hasn't been as smooth as we had hoped. We experienced a glitch in our automatic key delivery system that wasn't attaching keys to emails. Our engineering team are working quickly to rectify this. We're sorry if some of you may have received blank emails from us during this process.
If you have pre-purchased your Witcher 3 key from us, it should be with you within the next 24 hours. If for any reason your key is not with you by 22:59 UTC on Wednesday 20th May, OR if you are not satisfied with this service, we would like to offer you either a full refund, or 40% off your next purchase on Green Man Gaming. Again, we are sincerely sorry for any delay in getting your key to you, and we can assure you we are working hard to deliver the service you expect from us. We know it hasn't been the smoothest process and we will be evaluating what we could have done better for this launch to make sure our service to you in the future is the best it can be.
Thanks again for all of your patience, and apologies again for any inconvenience.
I have placed your ticket on hold and we will update you as soon as we have any news on this situation.
Green Man Gaming & Playfire
Customer Service
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Looks like they're working on it.