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 87.102.181.130