Fix broken encodeXtext()

client_test.go

@@ -931,3 +931,29 @@ Goodbye.`
 		t.Fatalf("QUIT failed: %s", err)
 	}
 }
+
+func TestClientXtext(t *testing.T) {
+	server := "220 hello world\r\n" +
+		"200 some more"
+	var wrote bytes.Buffer
+	var fake faker
+	fake.ReadWriter = struct {
+		io.Reader
+		io.Writer
+	}{
+		strings.NewReader(server),
+		&wrote,
+	}
+	c, err := NewClient(fake, "fake.host")
+	if err != nil {
+		t.Fatalf("NewClient: %v", err)
+	}
+	c.didHello = true
+	c.ext = map[string]string{"AUTH": "PLAIN"}
+	email := "[email protected]"
+	c.Mail(email, &MailOptions{Auth: &email})
+	c.Close()
+	if got, want := wrote.String(), "MAIL FROM:<[email protected]> [email protected]\r\n"; got != want {
+		t.Errorf("wrote %q; want %q", got, want)
+	}
+}

conn.go

@@ -426,16 +426,14 @@ func encodeXtext(raw string) string {
 	out.Grow(len(raw))
 
 	for _, ch := range raw {
-		if ch == '+' || ch == '=' {
+		switch {
+		case ch >= '!' && ch <= '~' && ch != '+' && ch != '=':
+			// printable non-space US-ASCII except '+' and '='
+			out.WriteRune(ch)
+		default:
 			out.WriteRune('+')
 			out.WriteString(strings.ToUpper(strconv.FormatInt(int64(ch), 16)))
 		}
-		if ch > '!' && ch < '~' { // printable non-space US-ASCII
-			out.WriteRune(ch)
-		}
-		// Non-ASCII.
-		out.WriteRune('+')
-		out.WriteString(strings.ToUpper(strconv.FormatInt(int64(ch), 16)))
 	}
 	return out.String()
 }