I have a map2n with dstnat rules that port forward from eth1 interface to the eth2 interface. I have no masquerade rule but the dstnat still works…I’m puzzled…
Wireshark trace on eth1 of map2n confirms that the reply traffic has been masqueraded back to the eth1 address. So is this a feature of dstnat that it automatically does src nat on the way back out?
Any advice much appreciated. 
I don’t know if this is relevant to your setup, but I don’t use masquerade either. I have 5 IP addresses from my ISP from a single connection to my cable modem, and I have set up 5 separate networks each directed to it’s own external IP address. Here is a sample of my setup:
/ip address
add address=10.26.5.1/24 comment=“Web-1” interface=“1 Web-1” network=10.26.5.0
add address=10.26.6.1/24 comment=“Web-2” interface=“2 Web-2” network=10.26.6.0
add address=10.26.7.1/24 comment=“Web-3” interface=“3 Web-3” network=10.26.7.0
add address=10.26.8.1/24 comment=“Web-4” interface=“4 Web-4” network=10.26.8.0
add address=10.26.3.1/24 comment=“Private” interface=“5 Private” network=10.26.3.0
add address=xxx.xxx.xxx.17/29 comment=“Web-1” interface=“12 Inet” network=xxx.xxx.xxx.16
add address=xxx.xxx.xxx.18/29 comment=“Web-2” interface=“12 Inet” network=xxx.xxx.xxx.16
add address=xxx.xxx.xxx.19/29 comment=“Web-3” interface=“12 Inet” network=xxx.xxx.xxx.16
add address=xxx.xxx.xxx.20/29 comment=“Web-4” interface=“12 Inet” network=xxx.xxx.xxx.16
add address=xxx.xxx.xxx.21/29 comment=“Private” interface=“12 Inet” network=xxx.xxx.xxx.16
Now instead of using masquerade, I am using the following nat rules so that each network is directed to the external IP address I want to associate it with:
/ip firewall nat
add action=src-nat chain=srcnat comment=“Web-1 nat to Wan17” src-address=10.26.5.0/24 to-addresses=xxx.xxx.xxx.17
add action=src-nat chain=srcnat comment=“Web-2 nat to Wan18” src-address=10.26.6.0/24 to-addresses=xxx.xxx.xxx.18
add action=src-nat chain=srcnat comment=“Web-3 nat to Wan19” src-address=10.26.7.0/24 to-addresses=xxx.xxx.xxx.19
add action=src-nat chain=srcnat comment=“Web-4 nat to Wan20” src-address=10.26.8.0/24 to-addresses=xxx.xxx.xxx.20
add action=src-nat chain=srcnat comment=“Private nat to Wan21” src-address=10.26.3.0/24 to-addresses=xxx.xxx.xxx.21
This was the only way I could get for example the 10.26.5.0 network to use only the xxx.xxx.xxx.17 external IP address.