As an aside, I’m a bit confused as to how a blacklist may be working.
A friend tried to send an email to me and they said it bounced. When I looked in my logs, it had indeed been rejected. The reason was a blacklist of an IPv6 address representing the mail server of the machine they’re on (a Bytemark one also, as it happens).
The blacklist service is s5h.net, yet when I run blacklist testing across the domain name of their email address, the mail server name, and the mail server IP (which is what was matched), all say there are no problems. This is within an hour of the issue being reported.
Why might this be?