How to set up SNAT for 4 addresses in iptables in Proxmox for a virtual machine with iredmail?

Question

linux

A server is rented from Hetzner, Proxmox is deployed on it with 5 white IPs (1.2.3.1 - 1.2.3.5).
By default, 1.2.3.5 is used to control the hypervisor itself, internal 192.168.0.1 for NAT of all virtual machines.
Iredmail is installed on a virtual machine with Ubuntu 18.04, 5 interfaces are configured on it via netplan (192.168.0.101 - 192.168.0.105). Interface 192.168.0.105 main with gateway 192.168.0.1. 192.168.0.101 - 192.168.0.104 are configured in iredmail for 4 domains.
In Proxmox, the following rules are configured for mail:

-A PREROUTING -d 1.2.3.1/32 -p tcp -m multiport --dports 25,587,110,995,143,993 -j DNAT --to-destination 192.168.0.101
-A PREROUTING -d 1.2.3.2/32 -p tcp -m multiport --dports 25,587,110,995,143,993 -j DNAT --to-destination 192.168.0.102
-A PREROUTING -d 1.2.3.3/32 -p tcp -m multiport --dports 25,587,110,995,143,993 -j DNAT --to-destination 192.168.0.103
-A PREROUTING -d 1.2.3.4/32 -p tcp -m multiport --dports 25,587,110,995,143,993 -j DNAT --to-destination 192.168.0.104
-A POSTROUTING -s 192.168.0.101/32 -j SNAT --to-source 1.2.3.1
-A POSTROUTING -s 192.168.0.102/32 -j SNAT --to-source 1.2.3.2
-A POSTROUTING -s 192.168.0.103/32 -j SNAT --to-source 1.2.3.3
-A POSTROUTING -s 192.168.0.104/32 -j SNAT --to-source 1.2.3.4
-A POSTROUTING -s 192.168.0.0/24 -o vmbr0 -j MASQUERADE

3 domains with IP 192.168.0.101 - 192.168.0.103 work fine through SNAT rules, but 192.168.0.104 is not routed.
If the rule

-A POSTROUTING -s 192.168.0.104/32 -j SNAT --to-source 1.2.3.4

put first in the chain, then the rest of the rules with SNAT stop working.
Masquerading for 192.168.0.105 and other virtual machines works.
Setting up 1.2.3.1 - 1.2.3.4 directly on a virtual machine with iredmail will not work, because these addresses are also used to forward non-mail ports to other virtual machines.
How to properly configure iptables so that SNAT for 192.168.0.104 will work?
The audience needs help, advice, constructive criticism.

Answer 1

Just from personal, fresh: netplan was written by some Pelevin mushroom characters ... in general, I join the crowds of people shouting "throw out netplan")
Perhaps the throwing action will already be enough for the iptables rules to work as expected