t-online has no SPF & DMARC & DKIM settings. but google try to maintain a reliable sender IP list for them.
if you forward a message which is original from t-online to gmail, if you don't have SRS enabled for this forwarding, gmail will just reject it. as far as I know, many ISP level forwardings have NO SRS setup, including Mail.ru, Freenet.de, and even Outlook. since there is no spf, no dkim for t-online, how google know it's coming from a valid IP? the only reason I can image is that google help maintain that a ip list for t-online itself. how do you think of this? Thanks & regards.