Add TestClient_SendErrorReset to validate SMTP error handling

This test checks the client's ability to handle SMTP reset errors when sending an email. It verifies the correct error type, ensures it is recognized as a permanent error, and confirms the correct message ID handling.

client_test.go

@@ -1663,6 +1663,68 @@ func TestClient_SendErrorDataWrite(t *testing.T) {
 	}
 }
 
+func TestClient_SendErrorReset(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	featureSet := "250-AUTH PLAIN\r\n250-8BITMIME\r\n250-DSN\r\n250 SMTPUTF8"
+	go func() {
+		if err := simpleSMTPServer(ctx, featureSet, true); err != nil {
+			t.Errorf("failed to start test server: %s", err)
+			return
+		}
+	}()
+	time.Sleep(time.Millisecond * 300)
+
+	message := NewMsg()
+	if err := message.From("valid-from@domain.tld"); err != nil {
+		t.Errorf("failed to set FROM address: %s", err)
+		return
+	}
+	if err := message.To("valid-to@domain.tld"); err != nil {
+		t.Errorf("failed to set TO address: %s", err)
+		return
+	}
+	message.Subject("Test subject")
+	message.SetBodyString(TypeTextPlain, "Test body")
+	message.SetMessageIDWithValue("this.is.a.message.id")
+
+	client, err := NewClient(TestServerAddr, WithPort(TestServerPort),
+		WithTLSPortPolicy(NoTLS), WithSMTPAuth(SMTPAuthPlain),
+		WithUsername("toni@tester.com"),
+		WithPassword("V3ryS3cr3t+"))
+	if err != nil {
+		t.Errorf("unable to create new client: %s", err)
+	}
+	if err = client.DialWithContext(context.Background()); err != nil {
+		t.Errorf("failed to dial to test server: %s", err)
+	}
+	if err = client.Send(message); err == nil {
+		t.Error("expected Send() to fail but didn't")
+	}
+
+	var sendErr *SendError
+	if !errors.As(err, &sendErr) {
+		t.Errorf("expected *SendError type as returned error, but got %T", sendErr)
+	}
+	if errors.As(err, &sendErr) {
+		if sendErr.IsTemp() {
+			t.Errorf("expected permanent error but IsTemp() returned true")
+		}
+		if sendErr.Reason != ErrSMTPReset {
+			t.Errorf("expected ErrSMTPReset error, but got %s", sendErr.Reason)
+		}
+		if !strings.EqualFold(sendErr.MessageID(), "<this.is.a.message.id>") {
+			t.Errorf("expected message ID: %q, but got %q", "<this.is.a.message.id>",
+				sendErr.MessageID())
+		}
+	}
+
+	if err = client.Close(); err != nil {
+		t.Errorf("failed to close server connection: %s", err)
+	}
+}
+
 // getTestConnection takes environment variables to establish a connection to a real
 // SMTP server to test all functionality that requires a connection
 func getTestConnection(auth bool) (*Client, error) {