smtp: Distinguish permanent errors

This patch makes the SMTP courier distinguish permanent errors, so the queue
can decide whether to retry or not.

It's based on the SMTP replies: 5xy is considerent permanent, anything else is
transient (including errors other than protocol issues).

internal/smtp/smtp_test.go

@@ -3,6 +3,7 @@ package smtp
 import (
 	"bufio"
 	"bytes"
+	"fmt"
 	"net"
 	"net/smtp"
 	"net/textproto"
@@ -11,6 +12,24 @@ import (
 	"time"
 )
 
+func TestIsPermanent(t *testing.T) {
+	cases := []struct {
+		err       error
+		permanent bool
+	}{
+		{&textproto.Error{499, ""}, false},
+		{&textproto.Error{500, ""}, true},
+		{&textproto.Error{599, ""}, true},
+		{&textproto.Error{600, ""}, false},
+		{fmt.Errorf("something"), false},
+	}
+	for _, c := range cases {
+		if p := IsPermanent(c.err); p != c.permanent {
+			t.Errorf("%v: expected %v, got %v", c.err, c.permanent, p)
+		}
+	}
+}
+
 func TestIsASCII(t *testing.T) {
 	cases := []struct {
 		str   string