Some of our emails are rejected with this message:
There was a SMTP communication problem with the recipient's email server. Please contact your system administrator.
<mail.example.org #5.5.0 smtp;554 <unknown[1xx.xxx.xx.x]>: Client host rejected: rDNS/DNS validation failed. Please setup matching DNS and rDNS records.
The email IP address resolves to mail.example.org and our main IP address resolves to host.example.org.
It looks like the receiving side of the email sees the email coming from the host IP address instead of the mail IP address, so because it cannot solve the DNS it rejects my email.
How should I solve that? Why do they see it coming from the host IP address instead of the mail IP address.