Adding WithDialContextFunc client option

client.go

@@ -76,6 +76,9 @@ const (
 	DSNRcptNotifyDelay DSNRcptNotifyOption = "DELAY"
 )
 
+// DialContextFunc is a type to define custom DialContext function.
+type DialContextFunc func(ctx context.Context, network, address string) (net.Conn, error)
+
 // Client is the SMTP client struct
 type Client struct {
 	// co is the net.Conn that the smtp.Client is based on
@@ -137,6 +140,9 @@ type Client struct {
 
 	// l is a logger that implements the log.Logger interface
 	l log.Logger
+
+	// dialContextFunc is a custom DialContext function to dial target SMTP server
+	dialContextFunc DialContextFunc
 }
 
 // Option returns a function that can be used for grouping Client options
@@ -401,6 +407,14 @@ func WithoutNoop() Option {
 	}
 }
 
+// WithDialContextFunc overrides the default DialContext for connecting SMTP server
+func WithDialContextFunc(f DialContextFunc) Option {
+	return func(c *Client) error {
+		c.dialContextFunc = f
+		return nil
+	}
+}
+
 // TLSPolicy returns the currently set TLSPolicy as string
 func (c *Client) TLSPolicy() string {
 	return c.tlspolicy.String()
@@ -481,18 +495,18 @@ func (c *Client) DialWithContext(pc context.Context) error {
 	ctx, cfn := context.WithDeadline(pc, time.Now().Add(c.cto))
 	defer cfn()
 
+	if c.dialContextFunc == nil {
 		nd := net.Dialer{}
+		c.dialContextFunc = nd.DialContext
 
-	var err error
 		if c.ssl {
 			td := tls.Dialer{NetDialer: &nd, Config: c.tlsconfig}
-
 			c.enc = true
-		c.co, err = td.DialContext(ctx, "tcp", c.ServerAddr())
+			c.dialContextFunc = td.DialContext
 		}
-	if !c.ssl {
-		c.co, err = nd.DialContext(ctx, "tcp", c.ServerAddr())
 	}
+	var err error
+	c.co, err = c.dialContextFunc(ctx, "tcp", c.ServerAddr())
 	if err != nil {
 		return err
 	}

client_test.go

@@ -9,6 +9,7 @@ import (
 	"crypto/tls"
 	"errors"
 	"fmt"
+	"net"
 	"os"
 	"strconv"
 	"strings"
@@ -108,6 +109,9 @@ func TestNewClientWithOptions(t *testing.T) {
 		{"WithoutNoop()", WithoutNoop(), false},
 		{"WithDebugLog()", WithDebugLog(), false},
 		{"WithLogger()", WithLogger(log.New(os.Stderr, log.LevelDebug)), false},
+		{"WithDialContextFunc()", WithDialContextFunc(func(ctx context.Context, network, address string) (net.Conn, error) {
+			return nil, nil
+		}), false},
 
 		{
 			"WithDSNRcptNotifyType() NEVER combination",
@@ -703,6 +707,31 @@ func TestClient_DialWithContextOptions(t *testing.T) {
 	}
 }
 
+// TestClient_DialWithContextOptionDialContextFunc tests the DialWithContext method plus
+// use dialContextFunc option for the Client object
+func TestClient_DialWithContextOptionDialContextFunc(t *testing.T) {
+	c, err := getTestConnection(true)
+	if err != nil {
+		t.Skipf("failed to create test client: %s. Skipping tests", err)
+	}
+
+	called := false
+	c.dialContextFunc = func(ctx context.Context, network, address string) (net.Conn, error) {
+		called = true
+		return (&net.Dialer{}).DialContext(ctx, network, address)
+	}
+
+	ctx := context.Background()
+	if err := c.DialWithContext(ctx); err != nil {
+		t.Errorf("failed to dial with context: %s", err)
+		return
+	}
+
+	if called == false {
+		t.Errorf("dialContextFunc supposed to be called but not called")
+	}
+}
+
 // TestClient_DialSendClose tests the Dial(), Send() and Close() method of Client
 func TestClient_DialSendClose(t *testing.T) {
 	if os.Getenv("TEST_ALLOW_SEND") == "" {