Add NetDialTimeout to Dialer for per Dialer proxying (i.e. Google's A…

smtp.go

@@ -47,6 +47,8 @@ type Dialer struct {
 	// Whether we should retry mailing if the connection returned an error,
 	// defaults to true.
 	RetryFailure bool
+	// Overrides NetDialTimeout for this dialer
+	NetDialTimeout func(network, address string, timeout time.Duration) (net.Conn, error)
 }
 
 // NewDialer returns a new SMTP Dialer. The given parameters are used to connect
@@ -79,7 +81,11 @@ var NetDialTimeout = net.DialTimeout
 // Dial dials and authenticates to an SMTP server. The returned SendCloser
 // should be closed when done using it.
 func (d *Dialer) Dial() (SendCloser, error) {
-	conn, err := NetDialTimeout("tcp", addr(d.Host, d.Port), d.Timeout)
+	ndt := NetDialTimeout
+	if d.NetDialTimeout != nil {
+		ndt = d.NetDialTimeout
+	}
+	conn, err := ndt("tcp", addr(d.Host, d.Port), d.Timeout)
 	if err != nil {
 		return nil, err
 	}