You can do it with one public interface only if you have set more then one address on it . Adding additional address will do the work but you need to have pool of public ip addresses.
I have 8 IP addresses, 5 of them are free for use.
Each additional address has assigned 1 of these public IPs, and DNAT does correctly send mails for company a to mail server 1 and those for company b to mail server 2.
So I think this is all okay.
My problem is that some of our mails are blocked by spam filters with reverse MX lookup because the header shows the IP 1.1.1.0/29
This IP address does and can not have a ptr record.
I managed to translate 1.1.1.0/29 to 1.1.1.2 or 1.1.1.3 with a SNAT rule, but it does not work to translate traffic from Host 192.168.1.1 to 1.1.1.2 and traffic from 192.168.2.2 to 1.1.1.3 with 2 SNAT rules.
I have 8 IP addresses, 5 of them are free for use.
Each additional address has assigned 1 of these public IPs, and DNAT does correctly send mails for company a to mail server 1 and those for company b to mail server 2.
So I think this is all okay.
My problem is that some of our mails are blocked by spam filters with reverse MX lookup because the header shows the IP 1.1.1.0/29
This IP address does and can not have a ptr record.
I managed to translate 1.1.1.0/29 to 1.1.1.2 or 1.1.1.3 with a SNAT rule, but it does not work to translate traffic from Host 192.168.1.1 to 1.1.1.2 and traffic from 192.168.2.2 to 1.1.1.3 with 2 SNAT rules.