what does one do if one's email to winner bounces?

make sure the email is correct as it is displayed her on the site. try to contact them via steam. id say if nothing works then ask support if you need to reroll or if they need to make him verify his email somehow

I don't know what you mean with "the mail failed" but try to connect him on steam next.

Add them on Steam and hand over the game via chat.

I'd advise doing this anyway, as you can make sure they receive the game, and be certain that everything has panned out ok.

I had this happen a month ago and chose to track the person down.

The email had a permanent delivery failure and the person did not have public comments on his Steam Profile. I fr requested him & after 2 days I noticed he had been online but did not accept the request. So at that point I started to VERY POLITELY go through his steam friends, being careful not to be annoying or sounding like a scammer. The 2nd person that responded to "yes, I talk to xxxx pretty often." got pissed off and didn't want to be bothered. But then I told him why I was reaching out to him, wanted to give the guy a free game he won on the site & his email bounced... The guy apologized and delivered the message. The winner accepted my fr and get the game the next day. It all worked out and those two got to see there are some decent people on steam (me). ;) I speak English and those two's primary language is Spanish (Spain). So there was a small language barrier, but it got worked out. It took 4 days to get him his game because he had closed himself off and not updated his SG account.

With all this in mind, you are not expected to do what I did. All you have to do is attempt to send an email and then reroll after 1 week. In my 24 ga, I have rerolled once. It was no big deal and was approved quick.

check their steam profile and add them as friend.

If they refuse your friend request and you can't leave a comment re-roll.

Email bounces = friend request through Steam. If I don't get a response after 24h, I'll request a reroll.

The advice given already pretty much covers the original question. For clarity, I'll reiterate.

1) Check the email address to be sure it's correct and try again. Make sure you include "SteamGifts" in the title.

At this point, if you receive no response, you may wait the full seven days and then ask for a re-roll. Your responsibility is over. If the winner has made it difficult or impossible for you to deliver the game, that is not your fault. If, however, you want to spend more time and effort on contacting the winner, then you can do the following:

2) Attempt to contact the winner through Steam chat. If that fails,
3) Attempt to contact the winner's friend through Steam chat. If that fails,
4) Edit the giveaway description so that it says something like "Winner not yet responding to email or Steam chat."
5) After seven days of trying to contact the winner, give up and ask Support for a re-roll.