My Server has 4 IP addresses.
(IPs are for example only) 126.96.36.199 188.8.131.52 184.108.40.206 220.127.116.11
Under IIS, I have a Virtual SMTP Server on 18.104.22.168, on port 25. The virtual server installed there at the start on 22.214.171.124 is disabled.
126.96.36.199 is mapped to my hostname, mailhost.mydomain.com and the rDNS PTR is configured for that as well.
When I send an Email using mailhost.mydomain.com - it works, but it shows up in headers that the email is coming from 188.8.131.52 with no reference to 184.108.40.206 at all.