Maybe Bluewin requires a matching PTR? Currently 188.8.131.52 resolves to
and not to mail.aerni.com <http://mail.aerni.com/>.
> Does anyone know, how bluewin does 'verify' an MX?
> The MX of our customer is prefectly reachable for sender verification
> but his emails sent 'to' bluewin recipients are getting blocked with:
> MAIL FROM: <[scrubbed]@aerni.com> Unable to verify MX-Record for
> domain aerni.com
> aerni.com mail is handled by 10 mail.aerni.com.
> mail.aerni.com has address 184.108.40.206
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