Make Dialer's domain configurable

The net/smtp library sends "localhost" unless otherwise instructed. This
commit adds a new "LocalName" field to the Dialer struct that allows
users to override what gets sent with SMTP's HELO command.

Fixes #40

smtp.go

@@ -24,6 +24,9 @@ type Dialer struct {
 	// TSLConfig represents the TLS configuration used for the TLS (when the
 	// STARTTLS extension is used) or SSL connection.
 	TLSConfig *tls.Config
+	// LocalName is the hostname sent to the SMTP server with the HELO command.
+	// By default, "localhost" is sent.
+	LocalName string
 }
 
 // NewPlainDialer returns a Dialer. The given parameters are used to connect to
@@ -77,6 +80,12 @@ func (d *Dialer) starttlsDial() (smtpClient, error) {
 		return nil, err
 	}
 
+	if d.LocalName != "" {
+		if err := c.Hello(d.LocalName); err != nil {
+			return nil, err
+		}
+	}
+
 	if ok, _ := c.Extension("STARTTLS"); ok {
 		if err := c.StartTLS(d.tlsConfig()); err != nil {
 			c.Close()
@@ -93,7 +102,18 @@ func (d *Dialer) sslDial() (smtpClient, error) {
 		return nil, err
 	}
 
-	return newClient(conn, d.Host)
+	c, err := newClient(conn, d.Host)
+	if err != nil {
+		return nil, err
+	}
+
+	if d.LocalName != "" {
+		if err := c.Hello(d.LocalName); err != nil {
+			return nil, err
+		}
+	}
+
+	return c, nil
 }
 
 func (d *Dialer) tlsConfig() *tls.Config {
@@ -164,6 +184,7 @@ var (
 )
 
 type smtpClient interface {
+	Hello(string) error
 	Extension(string) (bool, string)
 	StartTLS(*tls.Config) error
 	Auth(smtp.Auth) error

smtp_test.go

@@ -56,8 +56,10 @@ func TestDialerSSL(t *testing.T) {
 
 func TestDialerConfig(t *testing.T) {
 	d := NewPlainDialer(testHost, testPort, "user", "pwd")
+	d.LocalName = "test"
 	d.TLSConfig = testConfig
 	testSendMail(t, d, []string{
+		"Hello test",
 		"Extension STARTTLS",
 		"StartTLS",
 		"Extension AUTH",
@@ -75,8 +77,10 @@ func TestDialerConfig(t *testing.T) {
 
 func TestDialerSSLConfig(t *testing.T) {
 	d := NewPlainDialer(testHost, testSSLPort, "user", "pwd")
+	d.LocalName = "test"
 	d.TLSConfig = testConfig
 	testSendMail(t, d, []string{
+		"Hello test",
 		"Extension AUTH",
 		"Auth",
 		"Mail " + testFrom,
@@ -118,6 +122,11 @@ type mockClient struct {
 	config *tls.Config
 }
 
+func (c *mockClient) Hello(localName string) error {
+	c.do("Hello " + localName)
+	return nil
+}
+
 func (c *mockClient) Extension(ext string) (bool, string) {
 	c.do("Extension " + ext)
 	return true, ""