Gentlemen (and ladies!) - I'm not often stuck with no idea which way to even begin... but I am this afternoon.

I've a customer who has been trying to email a supplier of theirs, but the mail keeps bouncing back. It's not the usual fat-fingered address. When I started investigating it, I see the recipients email is handled by mx1 and

When I send an email (from a different domain to my customer) to the recipient I get an immediate bounce that says:

     ----- Transcript of session follows -----
  ... while talking to
  >>> RCPT To:<****@******>
  <<< 550 Sorry, we do not accept connections from you.
  550 5.1.1 <****@******>... User unknown
  451 4.4.1 reply: read error from

I've checked and my mailservers are not in any blacklist I can find.
I've never been used as a spam relay, or origin of spam, the mail server reputation on everything I've been able to find is "good". Yes, I have proper RDNS etc and have had for decades.

Both and are a waste of time.
whois doesn't return anything useful.
Google isn't returning anything helpful or relevant.

Anyone got any bright ideas?

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