Awesome
smtptest 
smtptest provides SMTP server for testing.
Usage
package main
import (
"fmt"
"net/smtp"
"strings"
"testing"
"github.com/k1LoW/smtptest"
)
const testMsg = "To: alice@example.net\r\n" +
"Subject: Hello Gophers!\r\n" +
"\r\n" +
"This is the email body.\r\n"
func TestSendMail(t *testing.T) {
ts, auth, err := smtptest.NewServerWithAuth()
if err != nil {
t.Fatal(err)
}
t.Cleanup(func() {
ts.Close()
})
addr := ts.Addr()
if err := smtp.SendMail(addr, auth, "sender@example.org", []string{"alice@example.net"}, []byte(testMsg)); err != nil {
t.Fatal(err)
}
if len(ts.Messages()) != 1 {
t.Errorf("got %v\nwant %v", len(ts.Messages()), 1)
}
msgs := ts.Messages()
got := msgs[0].Header.Get("To")
want := "alice@example.net"
if got != want {
t.Errorf("got %v\nwant %v", got, want)
}
}