To get outbound traffic from a server using 1:1 NAT, we use Manual Outbound NAT.
There will be a default mapping for "192.168.1.1/24" which is for any PC on the LAN, with its NAT address set to "WAN Address."
Above that one, add a row for 192.168.1.100/32 with a NAT address of the .42 public IP for this server. This will force outbound traffic, from that server only, to use the .42 address.