Problem with SNAT

I have Turris Omnia with operating system version 5.1.1.
I have been assigned two public IP addresses by my ISP. I would like to use one for normal operation and I would like to have the other reserved for mail server communication.

I wanted to solve translations from the internal network using SNAT. Unfortunately, even if I try to set even a simple SNAT, when I translate the entire internal range to one address, the translation will not take place.

Masquerade it works normally, but I won’t split the communication there.

Don’t know what I could do wrong?