diff --git a/smtp/smtp_test.go b/smtp/smtp_test.go
index 15ea7c5..931b74e 100644
--- a/smtp/smtp_test.go
+++ b/smtp/smtp_test.go
@@ -31,6 +31,7 @@ import (
 	"net"
 	"os"
 	"strings"
+	"sync"
 	"sync/atomic"
 	"testing"
 	"time"
@@ -2235,17 +2236,17 @@ func TestClient_Mail(t *testing.T) {
 		serverPort := int(TestServerPortBase + PortAdder.Load())
 		featureSet := "250-8BITMIME\r\n250 STARTTLS"
 		echoBuffer := bytes.NewBuffer(nil)
-		go func(buf *bytes.Buffer) {
-			if err := simpleSMTPServer(ctx, t, &serverProps{
-				EchoBuffer: buf,
-				FeatureSet: featureSet,
-				ListenPort: serverPort,
-			},
-			); err != nil {
+		props := &serverProps{
+			EchoBuffer: echoBuffer,
+			FeatureSet: featureSet,
+			ListenPort: serverPort,
+		}
+		go func() {
+			if err := simpleSMTPServer(ctx, t, props); err != nil {
 				t.Errorf("failed to start test server: %s", err)
 				return
 			}
-		}(echoBuffer)
+		}()
 		time.Sleep(time.Millisecond * 30)
 
 		client, err := Dial(fmt.Sprintf("%s:%d", TestServerAddr, serverPort))
@@ -2261,7 +2262,9 @@ func TestClient_Mail(t *testing.T) {
 			t.Errorf("failed to set mail from address: %s", err)
 		}
 		expected := "MAIL FROM:<valid-from@domain.tld> BODY=8BITMIME"
+		props.BufferMutex.RLock()
 		resp := strings.Split(echoBuffer.String(), "\r\n")
+		props.BufferMutex.RUnlock()
 		if !strings.EqualFold(resp[5], expected) {
 			t.Errorf("expected mail from command to be %q, but sent %q", expected, resp[5])
 		}
@@ -2273,17 +2276,17 @@ func TestClient_Mail(t *testing.T) {
 		serverPort := int(TestServerPortBase + PortAdder.Load())
 		featureSet := "250-SMTPUTF8\r\n250 STARTTLS"
 		echoBuffer := bytes.NewBuffer(nil)
-		go func(buf *bytes.Buffer) {
-			if err := simpleSMTPServer(ctx, t, &serverProps{
-				EchoBuffer: buf,
-				FeatureSet: featureSet,
-				ListenPort: serverPort,
-			},
-			); err != nil {
+		props := &serverProps{
+			EchoBuffer: echoBuffer,
+			FeatureSet: featureSet,
+			ListenPort: serverPort,
+		}
+		go func() {
+			if err := simpleSMTPServer(ctx, t, props); err != nil {
 				t.Errorf("failed to start test server: %s", err)
 				return
 			}
-		}(echoBuffer)
+		}()
 		time.Sleep(time.Millisecond * 30)
 
 		client, err := Dial(fmt.Sprintf("%s:%d", TestServerAddr, serverPort))
@@ -2299,7 +2302,9 @@ func TestClient_Mail(t *testing.T) {
 			t.Errorf("failed to set mail from address: %s", err)
 		}
 		expected := "MAIL FROM:<valid-from@domain.tld> SMTPUTF8"
+		props.BufferMutex.RLock()
 		resp := strings.Split(echoBuffer.String(), "\r\n")
+		props.BufferMutex.RUnlock()
 		if !strings.EqualFold(resp[5], expected) {
 			t.Errorf("expected mail from command to be %q, but sent %q", expected, resp[5])
 		}
@@ -2311,17 +2316,17 @@ func TestClient_Mail(t *testing.T) {
 		serverPort := int(TestServerPortBase + PortAdder.Load())
 		featureSet := "250-SMTPUTF8\r\n250 STARTTLS"
 		echoBuffer := bytes.NewBuffer(nil)
-		go func(buf *bytes.Buffer) {
-			if err := simpleSMTPServer(ctx, t, &serverProps{
-				EchoBuffer: buf,
-				FeatureSet: featureSet,
-				ListenPort: serverPort,
-			},
-			); err != nil {
+		props := &serverProps{
+			EchoBuffer: echoBuffer,
+			FeatureSet: featureSet,
+			ListenPort: serverPort,
+		}
+		go func() {
+			if err := simpleSMTPServer(ctx, t, props); err != nil {
 				t.Errorf("failed to start test server: %s", err)
 				return
 			}
-		}(echoBuffer)
+		}()
 		time.Sleep(time.Millisecond * 30)
 
 		client, err := Dial(fmt.Sprintf("%s:%d", TestServerAddr, serverPort))
@@ -2337,7 +2342,9 @@ func TestClient_Mail(t *testing.T) {
 			t.Errorf("failed to set mail from address: %s", err)
 		}
 		expected := "MAIL FROM:<valid-from+📧@domain.tld> SMTPUTF8"
+		props.BufferMutex.RLock()
 		resp := strings.Split(echoBuffer.String(), "\r\n")
+		props.BufferMutex.RUnlock()
 		if !strings.EqualFold(resp[5], expected) {
 			t.Errorf("expected mail from command to be %q, but sent %q", expected, resp[5])
 		}
@@ -2349,17 +2356,17 @@ func TestClient_Mail(t *testing.T) {
 		serverPort := int(TestServerPortBase + PortAdder.Load())
 		featureSet := "250-DSN\r\n250 STARTTLS"
 		echoBuffer := bytes.NewBuffer(nil)
-		go func(buf *bytes.Buffer) {
-			if err := simpleSMTPServer(ctx, t, &serverProps{
-				EchoBuffer: buf,
-				FeatureSet: featureSet,
-				ListenPort: serverPort,
-			},
-			); err != nil {
+		props := &serverProps{
+			EchoBuffer: echoBuffer,
+			FeatureSet: featureSet,
+			ListenPort: serverPort,
+		}
+		go func() {
+			if err := simpleSMTPServer(ctx, t, props); err != nil {
 				t.Errorf("failed to start test server: %s", err)
 				return
 			}
-		}(echoBuffer)
+		}()
 		time.Sleep(time.Millisecond * 30)
 
 		client, err := Dial(fmt.Sprintf("%s:%d", TestServerAddr, serverPort))
@@ -2376,7 +2383,9 @@ func TestClient_Mail(t *testing.T) {
 			t.Errorf("failed to set mail from address: %s", err)
 		}
 		expected := "MAIL FROM:<valid-from@domain.tld> RET=FULL"
+		props.BufferMutex.RLock()
 		resp := strings.Split(echoBuffer.String(), "\r\n")
+		props.BufferMutex.RUnlock()
 		if !strings.EqualFold(resp[5], expected) {
 			t.Errorf("expected mail from command to be %q, but sent %q", expected, resp[5])
 		}
@@ -2388,17 +2397,17 @@ func TestClient_Mail(t *testing.T) {
 		serverPort := int(TestServerPortBase + PortAdder.Load())
 		featureSet := "250-DSN\r\n250-8BITMIME\r\n250-SMTPUTF8\r\n250 STARTTLS"
 		echoBuffer := bytes.NewBuffer(nil)
-		go func(buf *bytes.Buffer) {
-			if err := simpleSMTPServer(ctx, t, &serverProps{
-				EchoBuffer: buf,
-				FeatureSet: featureSet,
-				ListenPort: serverPort,
-			},
-			); err != nil {
+		props := &serverProps{
+			EchoBuffer: echoBuffer,
+			FeatureSet: featureSet,
+			ListenPort: serverPort,
+		}
+		go func() {
+			if err := simpleSMTPServer(ctx, t, props); err != nil {
 				t.Errorf("failed to start test server: %s", err)
 				return
 			}
-		}(echoBuffer)
+		}()
 		time.Sleep(time.Millisecond * 30)
 
 		client, err := Dial(fmt.Sprintf("%s:%d", TestServerAddr, serverPort))
@@ -2415,7 +2424,9 @@ func TestClient_Mail(t *testing.T) {
 			t.Errorf("failed to set mail from address: %s", err)
 		}
 		expected := "MAIL FROM:<valid-from@domain.tld> BODY=8BITMIME SMTPUTF8 RET=FULL"
+		props.BufferMutex.RLock()
 		resp := strings.Split(echoBuffer.String(), "\r\n")
+		props.BufferMutex.RUnlock()
 		if !strings.EqualFold(resp[7], expected) {
 			t.Errorf("expected mail from command to be %q, but sent %q", expected, resp[7])
 		}
@@ -2490,17 +2501,17 @@ func TestClient_Rcpt(t *testing.T) {
 		serverPort := int(TestServerPortBase + PortAdder.Load())
 		featureSet := "250-DSN\r\n250 STARTTLS"
 		echoBuffer := bytes.NewBuffer(nil)
-		go func(buf *bytes.Buffer) {
-			if err := simpleSMTPServer(ctx, t, &serverProps{
-				EchoBuffer: buf,
-				FeatureSet: featureSet,
-				ListenPort: serverPort,
-			},
-			); err != nil {
+		props := &serverProps{
+			EchoBuffer: echoBuffer,
+			FeatureSet: featureSet,
+			ListenPort: serverPort,
+		}
+		go func() {
+			if err := simpleSMTPServer(ctx, t, props); err != nil {
 				t.Errorf("failed to start test server: %s", err)
 				return
 			}
-		}(echoBuffer)
+		}()
 		time.Sleep(time.Millisecond * 30)
 		client, err := Dial(fmt.Sprintf("%s:%d", TestServerAddr, serverPort))
 		if err != nil {
@@ -2519,7 +2530,9 @@ func TestClient_Rcpt(t *testing.T) {
 			t.Error("recpient address with newlines should fail")
 		}
 		expected := "RCPT TO:<valid-to@domain.tld> NOTIFY=SUCCESS"
+		props.BufferMutex.RLock()
 		resp := strings.Split(echoBuffer.String(), "\r\n")
+		props.BufferMutex.RUnlock()
 		if !strings.EqualFold(resp[5], expected) {
 			t.Errorf("expected rcpt to command to be %q, but sent %q", expected, resp[5])
 		}
@@ -2782,17 +2795,17 @@ func TestSendMail(t *testing.T) {
 		serverPort := int(TestServerPortBase + PortAdder.Load())
 		featureSet := "250-AUTH LOGIN\r\n250-DSN\r\n250 STARTTLS"
 		echoBuffer := bytes.NewBuffer(nil)
-		go func(buf *bytes.Buffer) {
-			if err := simpleSMTPServer(ctx, t, &serverProps{
-				EchoBuffer: buf,
-				FeatureSet: featureSet,
-				ListenPort: serverPort,
-			},
-			); err != nil {
+		props := &serverProps{
+			EchoBuffer: echoBuffer,
+			FeatureSet: featureSet,
+			ListenPort: serverPort,
+		}
+		go func() {
+			if err := simpleSMTPServer(ctx, t, props); err != nil {
 				t.Errorf("failed to start test server: %s", err)
 				return
 			}
-		}(echoBuffer)
+		}()
 		time.Sleep(time.Millisecond * 30)
 		addr := fmt.Sprintf("%s:%d", TestServerAddr, serverPort)
 		testHookStartTLS = func(config *tls.Config) {
@@ -2806,7 +2819,9 @@ func TestSendMail(t *testing.T) {
 			[]byte("test message")); err != nil {
 			t.Fatalf("failed to send mail: %s", err)
 		}
+		props.BufferMutex.RLock()
 		resp := strings.Split(echoBuffer.String(), "\r\n")
+		props.BufferMutex.RUnlock()
 		if len(resp)-1 != len(want) {
 			t.Fatalf("expected %d lines, but got %d", len(want), len(resp))
 		}
@@ -2857,17 +2872,17 @@ func TestSendMail(t *testing.T) {
 		serverPort := int(TestServerPortBase + PortAdder.Load())
 		featureSet := "250-AUTH LOGIN\r\n250-DSN\r\n250 STARTTLS"
 		echoBuffer := bytes.NewBuffer(nil)
-		go func(buf *bytes.Buffer) {
-			if err := simpleSMTPServer(ctx, t, &serverProps{
-				EchoBuffer: buf,
-				FeatureSet: featureSet,
-				ListenPort: serverPort,
-			},
-			); err != nil {
+		props := &serverProps{
+			EchoBuffer: echoBuffer,
+			FeatureSet: featureSet,
+			ListenPort: serverPort,
+		}
+		go func() {
+			if err := simpleSMTPServer(ctx, t, props); err != nil {
 				t.Errorf("failed to start test server: %s", err)
 				return
 			}
-		}(echoBuffer)
+		}()
 		time.Sleep(time.Millisecond * 30)
 		addr := fmt.Sprintf("%s:%d", TestServerAddr, serverPort)
 		testHookStartTLS = func(config *tls.Config) {
@@ -2887,7 +2902,9 @@ Goodbye.`)
 		if err := SendMail(addr, auth, "valid-from@domain.tld", []string{"valid-to@domain.tld"}, message); err != nil {
 			t.Fatalf("failed to send mail: %s", err)
 		}
+		props.BufferMutex.RLock()
 		resp := strings.Split(echoBuffer.String(), "\r\n")
+		props.BufferMutex.RUnlock()
 		if len(resp)-1 != len(want) {
 			t.Errorf("expected %d lines, but got %d", len(want), len(resp))
 		}
@@ -3542,6 +3559,7 @@ func testingKey(s string) string { return strings.ReplaceAll(s, "TESTING KEY", "
 
 // serverProps represents the configuration properties for the SMTP server.
 type serverProps struct {
+	BufferMutex     sync.RWMutex
 	EchoBuffer      io.Writer
 	FailOnAuth      bool
 	FailOnDataInit  bool
@@ -3640,9 +3658,11 @@ func handleTestServerConnection(connection net.Conn, t *testing.T, props *server
 			t.Logf("failed to write line: %s", err)
 		}
 		if props.EchoBuffer != nil {
-			if _, err := props.EchoBuffer.Write([]byte(data + "\r\n")); err != nil {
-				t.Errorf("failed write to echo buffer: %s", err)
+			props.BufferMutex.Lock()
+			if _, berr := props.EchoBuffer.Write([]byte(data + "\r\n")); berr != nil {
+				t.Errorf("failed write to echo buffer: %s", berr)
 			}
+			props.BufferMutex.Unlock()
 		}
 		_ = writer.Flush()
 	}
@@ -3665,9 +3685,11 @@ func handleTestServerConnection(connection net.Conn, t *testing.T, props *server
 		}
 		time.Sleep(time.Millisecond)
 		if props.EchoBuffer != nil {
+			props.BufferMutex.Lock()
 			if _, berr := props.EchoBuffer.Write([]byte(data)); berr != nil {
 				t.Errorf("failed write to echo buffer: %s", berr)
 			}
+			props.BufferMutex.Unlock()
 		}
 
 		var datastring string
@@ -3768,9 +3790,11 @@ func handleTestServerConnection(connection net.Conn, t *testing.T, props *server
 					break
 				}
 				if props.EchoBuffer != nil {
-					if _, err := props.EchoBuffer.Write([]byte(ddata)); err != nil {
-						t.Errorf("failed write to echo buffer: %s", err)
+					props.BufferMutex.Lock()
+					if _, berr := props.EchoBuffer.Write([]byte(ddata)); berr != nil {
+						t.Errorf("failed write to echo buffer: %s", berr)
 					}
+					props.BufferMutex.Unlock()
 				}
 				ddata = strings.TrimSpace(ddata)
 				if ddata == "." {