Ok, so they used a template on the paper, namely "{first}.{last}@hhi.fraunhofer.de", while the email addresses, if the names are applied to the template, do in fact yield valid email addresses.
It sounded as if they were using "john.doe@hhi.fraunhofer.de" while in reality it was an invalid email address ("because there’s no match with the email address"), that he would have tried to claim co-authorship via his "real" address, which might be something like "j.doe2@hhi.fraunhofer.de" (but luckily is not).
It's all clear now. Thank you for your explanation.
This is what I was asking about and I thank you and GP for clarifying the situation. There also send to be an unnecessary flamewar about the impermanence of email addresses generally, that's an unfortunate accident.
It sounded as if they were using "john.doe@hhi.fraunhofer.de" while in reality it was an invalid email address ("because there’s no match with the email address"), that he would have tried to claim co-authorship via his "real" address, which might be something like "j.doe2@hhi.fraunhofer.de" (but luckily is not).
It's all clear now. Thank you for your explanation.