Hi Achim,
the quick "fix" is to disable email notification for this build job.
The real problem is, when I set up the Jenkins server, I created a Google Mail account for email notifications from Jenkins, which never worked smoothly. I had to log into the account from the web UI every few days or so to make it work again.
A while ago, I received a message from Google saying that the account had been disabled for not being compliant with Terms of Service, with nothing but a link to their full ToS - they did not even mention the specific reason.
I think I may have checked the Google+ option when creating the account, and they may have objected to the fact that the account owner is not a natural person. But maybe they would have the same objections for a plain old email account.
To sum up, we'll have to set up a new email account, but it would be nice to do that without further trial and error, so if anyone knows a suitable provider (it doesn't have to be Google) that does not block this type of machine-generated email from Jenkins, then please let me know.