Some of these are NDR spam however I have set the Firewall to block NDR spam.
I have found that for the domain example.nl all bounce mails are blocked. So when I send to a non existent recipient I will not receive a NDR report.
How come, since this domain example.nl is in the smtp list?
In fact the
relay server that has been whitelisted (Smtp.isp.nl) corresponds to a
very large number of IP addresses, and it's not possible to whitelist
all of them. The mail of last test, for example,
has used IP xxx.xx.xx.xxx, which is in the range of IPs of Smtp.isp.nl.
A deeper look at the email message shows that the email has crossed
these relay server before being returned back:
So, instead of whitelisting the server Smtp.example.nl, I have whitelisted
MS.example.local, and the result has been that the mail has been
recognized as valid and not as spam.
Now, please try to send again the same test mail, that, if the server
MS.example.local is in the path to the destination server, should
return back and be recognized as non-spam.