Running TrueNAS SCALE 25.04.01
I followed the documentation for scale but still had an error “Unable to find the server at” when I tried to send a test message.
docs/scale/scaletutorials/systemsettings/general/settingupsystememail/
I found a Reddit thread that solved my issue, but I will not link it here.
I needed to go to Network → Global Configurations → Settings and add my router IP to the IPv4 Default Gateway AND as of one the Nameservers. I am running Unbound with OpnSense on my router, so this sort of makes sense.
Hope this helps someone!