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.

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