I'm receiving a lot of 421 rejects with:

Unexpected connection response from server:
421 mails from refused: local dynamic IP address"

Does anybody recognize the text of the message? I'd like to confirm that
there are no popular DUL lists showing as dynamic, but the 421
message says very little. DNSReport says it's clean. I've tried to contact
some postmaster accounts (using Yahoo.com, since I can't use my own mailer)
but they appear to be RFC ignorant too. Unfortunately, it's not just one ISP
in Germany and I'd like to understand if there's anything I can do on my

The only thing that comes to mind is that my rDNS is delegated to my own
name server. Maybe there's some sort of DNS software out in the wild that
doesn't support delegation? I'm really at a loss.