mail.go - test you mailserver sender restrictions (and other things) without the need of configuring a mail client, and without the hassle of a telnet-transaction...

mail.go

package main

import (
        "log"
        "net/smtp"
        "flag"
        "fmt"
        "os"
)

func main() {
        // ${0} [flags] <sender> <recipient>
        // flags and arguments
        server := flag.String("server", "mail.example.com", "The SMTP server to test.")
        serverPort := flag.Int("port", 25, "The SMTP port to use")

        username := flag.String("user", "", "The SMTP username")
        password := flag.String("password", "", "The SMTP user's password")

        flag.Parse()

        args := flag.Args()

        if len(args) < 2 {
                fmt.Fprint(os.Stderr, "Usage: mail [flags] <sender> <recipient>")
                os.Exit(1)
        }
        // Set up authentication information.
        auth := smtp.PlainAuth(
                "",
                *username,
                *password,
                *server,
        )
        // Connect to the server, authenticate, set the sender and recipient,
        // and send the email all in one step.
        err := smtp.SendMail(
                fmt.Sprint(*server, ":", *serverPort),
                auth,
                args[0],
                []string{args[1]},
                []byte("This is a test mail."),
        )
        if err != nil {
                log.Fatal(err)
        }
}