Thanks
UFHH01 for your nice answer.
I tried a few things but still no solution ... and I found out something really strange.
I will inform STRATO about "reverse dns entry for your IP to your desired hostname" but as far as I know the VPS is "as is" and they don't do any configuration.
I tried to add the STRATO hostname to my spf-record by adding "a:h12345.stratoserver.net" however this didn't change anything. I actually don't think that spf is the problem, because the source of my mail messages says that spf-test passed. So i don't think that that is the problem.
Now here comes the strange thing ... and believe me ... I think it is really strange. There is one situation that all mail ends up in the inbox like it is supposed to. Do you know when? When I set my From address and return-path to the mail address that I used to register my domain (as administrative and technical contact). When I use that mail address (something like
[email protected]) all mail ends up in the inbox as supposed to, but whenever I try any other name, for example
[email protected],
[email protected],
[email protected] and so on, it ends up in the spam folder.
How can this be? It seems like the spam filter checks the administrative or technical contact, notices the match and declares the mail as legit. But with any other name it is marked as spam. How can this be? I think this is so strange! By the way ... it's a new VPS, there isn't a live website on my server yet and I'm the only one with access. So it's impossible that it's on a blacklist.
The only thing that makes the difference between spam or no-spam is the name I use to send the mail.
[email protected] => to inbox. Any other name: spam.
Any ideas about what is going on? Every thought/idea is welcome ...