diff --git a/smtp.go b/smtp.go
index b523f15..2aa49c8 100644
--- a/smtp.go
+++ b/smtp.go
@@ -55,15 +55,10 @@ func NewPlainDialer(host string, port int, username, password string) *Dialer {
 	return NewDialer(host, port, username, password)
 }
 
-// NetDialTimeout specifies the DialTimeout function to establish a connection
-// to the SMTP server. This can be used to override dialing in the case that a
-// proxy or other special behavior is needed.
-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), 10*time.Second)
+	conn, err := netDialTimeout("tcp", addr(d.Host, d.Port), 10*time.Second)
 	if err != nil {
 		return nil, err
 	}
@@ -187,8 +182,9 @@ func (c *smtpSender) Close() error {
 
 // Stubbed out for tests.
 var (
-	tlsClient     = tls.Client
-	smtpNewClient = func(conn net.Conn, host string) (smtpClient, error) {
+	netDialTimeout = net.DialTimeout
+	tlsClient      = tls.Client
+	smtpNewClient  = func(conn net.Conn, host string) (smtpClient, error) {
 		return smtp.NewClient(conn, host)
 	}
 )
diff --git a/smtp_test.go b/smtp_test.go
index 3323029..b6f9155 100644
--- a/smtp_test.go
+++ b/smtp_test.go
@@ -248,7 +248,7 @@ func doTestSendMail(t *testing.T, d *Dialer, want []string, timeout bool) {
 		timeout: timeout,
 	}
 
-	NetDialTimeout = func(network, address string, d time.Duration) (net.Conn, error) {
+	netDialTimeout = func(network, address string, d time.Duration) (net.Conn, error) {
 		if network != "tcp" {
 			t.Errorf("Invalid network, got %q, want tcp", network)
 		}