This might be a bit hard for me to explain - and it is a pretty individual situation.
I got a native server at Hetzner (www.hetzner.de). The public IP is 88.[...].12. I got ESXi running on this server. I can access the esxi console by the public ip, but none of the virtual machines. That's why I bought a public subnet with 8 (6 usable) IPs (46.[...]) and an additional public ip (88.[...].26). This additional public ip belongs to the first virtual maschine - a firewall appliance - which is connected to the WAN. This need to be done this way - since it is the official way by hetzner. My 46. subnet is behind the firewall.
I got a virtualmin server with dovecot imap/pop3 server. When sending a email, most provider (gmail) will accept those mails, but a lot will put it into spam (aol).
My theory is: The MX line of my domain says of course the ip of the virtual machine (46.[...]), but in the raw email it says that email is sent by the ip of the firewall (88.[...].26), which doesnt sound trustworthy.
A solution would be if the firewall could handle mail, but it simply cant. How can I prevent this problem?