Blocking mail I can't answer / 2021-11-15

Someone mailed me a few days ago with an interesting question. So I typed a reasonably long answer. But upon sending this answer I received the following error message:

   ----- The following addresses had permanent fatal errors -----
<????????@outlook.com>
    (reason: 550 5.7.1 Unfortunately, messages from [45.83.232.134] weren't sent. Please contact your Internet se...ail.live.com/mail/troubleshooting.aspx#errors. [HE1EUR04FT003.eop-eur04.prod.protection.outlook.com])

   ----- Transcript of session follows -----
... while talking to outlook-com.olc.protection.outlook.com.:
>>> MAIL From:<?????? .at. idefix.net> SIZE=4837 BODY=8BITMIME
<<< 550 5.7.1 Unfortunately, messages from [45.83.232.134] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3150). You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors. [HE1EUR04FT003.eop-eur04.prod.protection.outlook.com]
554 5.0.0 Service unavailable

Trying to get my IPv4 address allowed didn't work. The form for getting IP addresses whitelisted did not allow for IPv6 addresses, but then again outlook.com has no IPv6 addresses listed for its MX record. I would think microsoft would do something with IPv6 to support innovations in Internet but I guess they only do that to win contracts.

After a while I got a response with a ticket number and an hour later a response that looked like maybe a person had taken a look at it, with "Our investigation has determined that the above IP(s) do not qualify for mitigation." So that leaves me with possible mails from outlook[.]com that I can't answer, making me look bad because I don't seem to reply at all.

I'm convinced the mail setup is correct on my end. The domain idefix.net has an SPF record and the mail was sent out via the approved route.

The only solution I can think of at the moment is blocking mail from outlook.com at the protocol level with an error message pointing at a webpage what the problem is, so when someone sends an e-mail from outlook[.]com to one of my domains they will get an error message with an embedded hint what they should do, namely We cannot reply to your mail, please send us mail from a different domain, see https://idefix.net/mailreject.html for an explanation. About the same as microsoft does, although the careful reader might have noticed the error code S3150 is not mentioned at http://mail.live.com/mail/troubleshooting.aspx#errors.