From 63ae6fcf3028f21736fc79ccdb021ab1e237b881 Mon Sep 17 00:00:00 2001
From: wzshiming <wzshiming@foxmail.com>
Date: Mon, 3 Jun 2019 23:39:09 +0800
Subject: [PATCH] Add ProxyDial to specify optional dialing to establish a
 transport connection.

---
 smtp.go | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/smtp.go b/smtp.go
index 547e04d..5793ec9 100644
--- a/smtp.go
+++ b/smtp.go
@@ -1,6 +1,7 @@
 package mail
 
 import (
+	"context"
 	"crypto/tls"
 	"fmt"
 	"io"
@@ -12,6 +13,9 @@ import (
 
 // A Dialer is a dialer to an SMTP server.
 type Dialer struct {
+	// ProxyDial specifies the optional dial function for
+	// establishing the transport connection.
+	DialProxy func(ctx context.Context, network, address string) (net.Conn, error)
 	// Host represents the host of the SMTP server.
 	Host string
 	// Port represents the port of the SMTP server.
@@ -53,6 +57,7 @@ type Dialer struct {
 // to the SMTP server.
 func NewDialer(host string, port int, username, password string) *Dialer {
 	return &Dialer{
+		DialProxy:    (&net.Dialer{}).DialContext,
 		Host:         host,
 		Port:         port,
 		Username:     username,
@@ -79,7 +84,8 @@ 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)
+	ctx, _ := context.WithTimeout(context.Background(), d.Timeout)
+	conn, err := d.DialProxy(ctx, "tcp", addr(d.Host, d.Port))
 	if err != nil {
 		return nil, err
 	}