Hi Johannes,
Gone are the days of being able to send normal emails to these
providers. They use automated algorithms that block your email
automatically. Your IP reputation has a lot to do with it as well, along
with the IPs in your /24. For example, looking up your IP reputation, I
see that this /24 has several mail providers, so while you might not be
sending the actual spam, other IPs in your /24. Also, forwarded email
can have the same impact, as you are forwarding spam, and not
necessarily sending it directly.
See also:
https://www.talosintelligence.com/reputation_center/lookup?search=37.120.172.118
37.120.172.166 mail.digi-media-net.de Yes 0.0 1.7 No Poor
You might need to use one of the commercial providers, such as Amazon or
Sendgrid, to send your outbound emails, since the large providers will
not block them as easily.
Matthew
