Thanks for the info. I confirmed at least two people who did not receive any such e-mail. One is a Google Mail address (@gmail.com), the other one has a big German university's e-mail address. The mails are not in the spam folders either.

Just saying. Get a decent way to send them, theymos, and send all of them again. You cannot just set up a random server with a random IP address and send mails. It's not the 80's any more. Due to spam epidemic, major mail providers will reject those mails.