There are plenty of articles concerning postfix servers and multiple domains, relays and smarthost however they don't seem to address the architecture I have.
I have built a small network consisting of 3 Debian Jessie servers for the sake of this question Servers A, B & C, they are all on a local network with 192.168.x.x IP addresses.
Server A is the only web facing server and hosts web and email services. Email is configured to relay through an external (gmail) server - until I move to a proper domain and is stable and working fine for both incoming/outgoing email originated on Server A.
Servers B & C run other services such as database and one of them (server C) is running Nagios host/service monitoring.
My ultimate goal is to have Nagios mail out service update notifications and to do this it will need to send emails via Server A as that is the only internet facing server.
My assumption is that I will need to have postfix running on Server C and somehow configure it to send mail via Server A, likewise for Server A to accept mail from Server C for forwarding.
Is my assumption correct and if show how do I configure the servers to do this?
If my assumption is incorrect, what is the proper way of doing this?