Well your host service is listening on localhost aka 127.0.0.1 so no you won't be able to connect, you need to make it listen to 10.200.10.1/24 given the link on your last message. That would create the ip 10.200.10.1 on the loopback giving it access to your docker.
Ofc in your case you most likely want the ip to be something like 172.18.0.1 or some other IP not in use.
On a previous message you said you were able to access the host at 192.168.100.101 so instead of doing the above steps you can just add that IP to your postfix config and configure mattermost smtp to use it and again do the telnet below to ensure you can connect.
Also you might need to ensure the firewall is not blocking access to/from those ports on both. Some times by default the firewall will be blocking it..
So ping -c1 host, if u get the pings you can reach it, then telnet host 25 or 587 depending on what you want...