Not all combinations of these parameters are considered legal by the go smtp library 
You'll need a stack trace to go on further. Search in the logs or try to execute the -version command with the CLI, which is incidentally the complete server, as described here
I'll bet my yesterdays socks that there is some null pointer panic and connectToSMTPServer or newSMTPClient is involved. There is definitely at least one null pointer check missing there. 