The T-Online mail servers are currently on several SPAM blacklists, which is why mails from T-Online servers are currently often declared as SPAM or rejected directly. So if you are a T-Online customer and possibly use the mail servers as Exchange Smarthost, you should switch to another provider if possible.
The whole article can be found on Heise.de:
http://www.heise.de/newsticker/meldung/T-Online-Mailserver-von-Blacklistings-betroffen-1341226.html
