This is really quite fascinating. There is exactly one ISP that is blocking mail from the mail server I run, which is used to deliver mail for infosec.exchange - t-online.de. It annoys me to no end that I see people try to subscribe to infosec.exchange and the confirmation emails are rejected. So, I contacted their postmaster, and to their credit, they got back to me very quickly, but they are basically saying "yeah, you need to use one of the big mail providers if you want to send mail to t-online subscribers".... Ok then. My apologies to any t-online subscribers (who probably can't see this anyhow), but you'll apparently need to use a different email account to register.

@jerry DTAG are renowned in ISP circles for having a sense of self importance as corporate policy. :|

I get that some people might have no choice to use their broadband but genuinely stunned anyone is using their freebie email accounts in any serious capacity.

@interpipes @jerry As an ISP (and from the consumer side) they are quite good in Germany. I have not had any big interruptions in service whereas friends and colleagues with other ISPs (especially Vodafone) had many outages.
@AUROnline @jerry unfortunately from the perspective of other networks who have content DTAG users want to access, DTAG have the opinion that not only should their customers pay them, but everyone else on the Internet should pay them also and they sandbag other attempts to maintain sufficient capacity to get packets to eyeballs, apparently banking on the fact that DTAG users either have no choice or will blame the content providers who haven’t paid DTAGs toll.