Go package, based heavily on emersion's go-gmtp, with increased transparency of response codes and no sasl dependency. The purpose of this is to provide functions that act as a server to receive SMTP messages from your downstream client. These SMTP messages are relayed through to an upstream server.
The command / response exchanges are passed on transparently.
STARTTLS can be offered to the downstream client if you configure a valid certificate/key pair.
STARTTLS can be requested from the upstream server.
Line splitting functions are included for base64 encoded email handling by your app.
Get this project with go get github.com/tuck1s/go-smtpproxy.
cmd/proxy contains an example command-line app using this library:
cd cmd/proxy
go build
./proxy -h
SMTP proxy that accepts incoming messages from your downstream client, and relays on to an upstream server.
Usage of ./proxy:
-certfile string
Certificate file for this server
-downstream_debug string
File to write downstream server SMTP conversation for debugging
-in_hostport string
Port number to serve incoming SMTP requests (default "localhost:587")
-insecure_skip_verify
Skip check of peer cert on upstream side
-logfile string
File written with message logs (also to stdout)
-out_hostport string
host:port for onward routing of SMTP requests (default "smtp.sparkpostmail.com:587")
-privkeyfile string
Private key file for this server
-verbose
print out lots of messages