Propagate the error to the client if the external command fails

[ca0s]

This change makes the external command code return a generic 554 error to the SMTP client if the external command fails, so it knows sending failed and can retry.

[decke]

One last question is which SMTP status code to use in this case. 5xx codes will tell the client that the issue is permanent so do not try again. 451 sounds more appropriate and will indicate a temporary error.

[ca0s]

I chose 514 because in my specific use case (my external command sends the email through an HTTP API) it is actually a permanent error: it only fails if the message body is too big.

I don't have any issues changing it to 451, as my clients will react the same.

Would you be OK if I sent another PR defining a list of status codes for external commands so smtprelay can choose the best SMTP status code? Something like:

command status code	SMTP status code
0	250, mail sent
-1	451, temporary error, try again
-2	554, generic permanent error
-3	554 5.3.4, permanent error, message too big

Kind regards, and thank you for writing this tool.

[decke]

Instead of defining a list I'd prefer to leave this to the external command. So the external commands should return SMTP status codes as error codes. We won't be able to pass along nice error texts though.

[ca0s]

That would be the cleanest solution. However, UNIX does not allow process return codes outside of the range [0,255]. Returning values out of that range yields unexpected codes in a standard Linux box:

javi@pwn ~/test/go $ cat return_fixed.go
package main

import "os"

func main() {
        os.Exit(514)
}
javi@pwn ~/test/go $ cat exec.go
package main

import (
        "fmt"
        "os/exec"
        "os"
)

func main() {
        cmd := exec.Command(os.Args[1])
        err := cmd.Run()
        exitcode := cmd.ProcessState.ExitCode()
        fmt.Printf("err: %s, exitcode: %d\n", err, exitcode)
}
javi@pwn ~/test/go $ ./exec ./return_fixed
err: exit status 2, exitcode: 2

Therefore, I'm afraid some mapping will be required.

[decke]

Thanks a lot for your contribution!