Mad3: I have collected all stamps and I got the mail after some minutes.
Same here. Received the email like two or three minutes after collecting the last stamp.
Mad3: I have never gifted anything nor did I ever get a gift.
How does this work?
In this case there's two options:
a) Just give someone the code that you see in the email. He can then click on "redeem code" at the bottom of the website and enter it there.
b) Directly copy and paste the URL that the code and "redeem now" in your email lead to (just use "copy link location" if you're using an email client). The person following it will then be automatically lead to the "redeem code" part of the website with your code already entered, so he/she will only have to confirm if he/she is already logged in.