diff --git a/client.go b/client.go
index 21d265cf..02bfc3df 100644
--- a/client.go
+++ b/client.go
@@ -5,6 +5,7 @@
 package smtp
 
 import (
+	"bytes"
 	"crypto/tls"
 	"encoding/base64"
 	"errors"
@@ -150,30 +151,22 @@ func NewClientLMTP(conn net.Conn) *Client {
 
 // setConn sets the underlying network connection for the client.
 func (c *Client) setConn(conn net.Conn) {
-	c.conn = conn
-
-	var r io.Reader = conn
-	var w io.Writer = conn
-
-	r = &lineLimitReader{
+	r := &lineLimitReader{
 		R: conn,
 		// Doubled maximum line length per RFC 5321 (Section 4.5.3.1.6)
 		LineLimit: 2000,
 	}
 
-	r = io.TeeReader(r, clientDebugWriter{c})
-	w = io.MultiWriter(w, clientDebugWriter{c})
-
-	rwc := struct {
+	c.conn = conn
+	c.text = textproto.NewConn(struct {
 		io.Reader
 		io.Writer
 		io.Closer
 	}{
-		Reader: r,
-		Writer: w,
+		Reader: io.TeeReader(r, clientDebugWriter{[]byte("SERVER  "), c}),
+		Writer: io.MultiWriter(conn, clientDebugWriter{[]byte("CLIENT  "), c}),
 		Closer: conn,
-	}
-	c.text = textproto.NewConn(rwc)
+	})
 }
 
 // Close closes the connection.
@@ -942,14 +935,37 @@ func toSMTPErr(protoErr *textproto.Error) *SMTPError {
 }
 
 type clientDebugWriter struct {
-	c *Client
+	prefix []byte
+	c      *Client
 }
 
 func (cdw clientDebugWriter) Write(b []byte) (int, error) {
 	if cdw.c.DebugWriter == nil {
 		return len(b), nil
 	}
-	return cdw.c.DebugWriter.Write(b)
+
+	// Prefix every line with the prefix.
+	var n int
+	for {
+		i := bytes.Index(b, []byte("\r\n"))
+		if i == -1 {
+			i = len(b) - 1
+		}
+		_, err := cdw.c.DebugWriter.Write(cdw.prefix)
+		if err != nil {
+			return n, err
+		}
+		nn, err := cdw.c.DebugWriter.Write(b[:i+2])
+		if err != nil {
+			return n + nn, err
+		}
+		n += nn
+		b = b[i+2:]
+		if len(b) == 0 {
+			break
+		}
+	}
+	return n, nil
 }
 
 // validateLine checks to see if a line has CR or LF.
diff --git a/client_test.go b/client_test.go
index 6f35a7d6..5f586127 100644
--- a/client_test.go
+++ b/client_test.go
@@ -1148,3 +1148,44 @@ func TestClientMTPRIORITY(t *testing.T) {
 		t.Errorf("wrote %q; want %q", actualcmds, client)
 	}
 }
+
+func TestClientDebugWriter(t *testing.T) {
+	server := strings.Join(strings.Split(mtPriorityServer, "\n"), "\r\n")
+	client := strings.Join(strings.Split(mtPriorityClient, "\n"), "\r\n")
+
+	var (
+		wrote bytes.Buffer
+		dbg   = new(bytes.Buffer)
+		fake  faker
+	)
+	fake.ReadWriter = struct {
+		io.Reader
+		io.Writer
+	}{
+		strings.NewReader(server),
+		&wrote,
+	}
+
+	c := NewClient(fake)
+	c.DebugWriter = dbg
+	c.didHello = true
+	c.ext = map[string]string{"MT-PRIORITY": ""}
+	priority := 6
+	c.Rcpt("root@nsa.gov", &RcptOptions{
+		MTPriority: &priority,
+	})
+	c.Close()
+	if actualcmds := wrote.String(); client != actualcmds {
+		t.Errorf("wrote %q; want %q", actualcmds, client)
+	}
+
+	tr := strings.NewReplacer("\t", "", "\n", "\r\n")
+	want := tr.Replace(`
+		CLIENT  RCPT TO:<root@nsa.gov> MT-PRIORITY=6
+		SERVER  220 hello world
+		SERVER  250 ok
+	`[1:])
+	if dbg.String() != want {
+		t.Errorf("debug wrong\nhave: %q\nwant: %q", dbg, want)
+	}
+}