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.