diff --git a/client.go b/client.go
index f8805c6..b187cc9 100644
--- a/client.go
+++ b/client.go
@@ -11,6 +11,7 @@ package onlyoffice
 
 import (
 	"net/http"
+	"net/http/cookiejar"
 	"os"
 	"strings"
 )
@@ -35,8 +36,9 @@ type Client struct {
 
 // NewClient returns a new Client backed by http.DefaultClient.
 func NewClient(c Credentials) *Client {
+	jar, _ := cookiejar.New(nil)
 	return &Client{
-		client:      http.DefaultClient,
+		client:      &http.Client{Jar: jar},
 		credentials: &c,
 	}
 }
diff --git a/mails.go b/mails.go
index 5689c6c..491f725 100644
--- a/mails.go
+++ b/mails.go
@@ -7,7 +7,9 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"io"
 	"net/mail"
+	"net/http"
 	"net/url"
 	"strconv"
 	"strings"
@@ -96,6 +98,38 @@ func (c *Client) GetMailMessage(ctx context.Context, messageID string) (map[stri
 	return c.ResponseObject(ctx, "/api/2.0/mail/messages/"+url.PathEscape(id))
 }
 
+// DownloadMailAttachment fetches raw attachment bytes by mail attachment id via
+// the mail addon's download.ashx handler. This path relies on the session
+// cookie captured during authentication, so NewClient configures a cookie jar.
+func (c *Client) DownloadMailAttachment(ctx context.Context, attachmentID string) ([]byte, error) {
+	id := strings.TrimSpace(attachmentID)
+	if id == "" {
+		return nil, fmt.Errorf("DownloadMailAttachment: attachment id is required")
+	}
+	auth, err := c.authHeader()
+	if err != nil {
+		return nil, err
+	}
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, c.baseURL()+"/addons/mail/httphandlers/download.ashx?attachid="+url.QueryEscape(id), nil)
+	if err != nil {
+		return nil, err
+	}
+	req.Header.Set("Authorization", auth)
+	resp, err := c.client.Do(req)
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+	raw, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, err
+	}
+	if resp.StatusCode >= 400 {
+		return nil, fmt.Errorf("DownloadMailAttachment %s: %d %s", id, resp.StatusCode, truncate(string(raw), 400))
+	}
+	return raw, nil
+}
+
 // RemoveMailMessages deletes messages by id (PUT /api/2.0/mail/messages/remove).
 // The API response "response" field may be a number or object; success is HTTP 2xx.
 func (c *Client) RemoveMailMessages(ctx context.Context, ids ...int) (map[string]any, error) {
diff --git a/mails_test.go b/mails_test.go
index be81255..86b2374 100644
--- a/mails_test.go
+++ b/mails_test.go
@@ -1,8 +1,13 @@
 package onlyoffice
 
 import (
+	"context"
+	"net/http"
+	"net/http/cookiejar"
+	"net/http/httptest"
 	"strings"
 	"testing"
+	"time"
 )
 
 func TestResolveMailFolder(t *testing.T) {
@@ -97,3 +102,60 @@ func TestInt64FromMap(t *testing.T) {
 		t.Fatal("string")
 	}
 }
+
+func TestNewClientSetsCookieJar(t *testing.T) {
+	c := NewClient(Credentials{Url: "https://example.test", User: "u", Password: "p"})
+	if c.client == nil {
+		t.Fatal("client is nil")
+	}
+	if c.client.Jar == nil {
+		t.Fatal("cookie jar is nil")
+	}
+	if _, ok := c.client.Jar.(*cookiejar.Jar); !ok {
+		t.Fatalf("unexpected jar type %T", c.client.Jar)
+	}
+}
+
+func TestDownloadMailAttachmentUsesAuthCookie(t *testing.T) {
+	var gotAuth, gotCookie, gotPath string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		switch r.URL.Path {
+		case "/api/2.0/authentication.json":
+			http.SetCookie(w, &http.Cookie{Name: "sessionid", Value: "abc123", Path: "/"})
+			w.Header().Set("Content-Type", "application/json")
+			_, _ = w.Write([]byte(`{"response":{"token":"tok","expires":"2099-01-01T00:00:00.0000000+00:00"}}`))
+		case "/addons/mail/httphandlers/download.ashx":
+			gotAuth = r.Header.Get("Authorization")
+			gotCookie = r.Header.Get("Cookie")
+			gotPath = r.URL.RequestURI()
+			if gotCookie == "" {
+				http.Error(w, "missing cookie", http.StatusUnauthorized)
+				return
+			}
+			_, _ = w.Write([]byte("payload"))
+		default:
+			http.NotFound(w, r)
+		}
+	}))
+	defer srv.Close()
+
+	c := NewClient(Credentials{Url: srv.URL, User: "u", Password: "p"})
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	body, err := c.DownloadMailAttachment(ctx, "42")
+	if err != nil {
+		t.Fatalf("DownloadMailAttachment: %v", err)
+	}
+	if string(body) != "payload" {
+		t.Fatalf("body = %q", body)
+	}
+	if gotAuth != "tok" {
+		t.Fatalf("auth header = %q", gotAuth)
+	}
+	if !strings.Contains(gotCookie, "sessionid=abc123") {
+		t.Fatalf("cookie header = %q", gotCookie)
+	}
+	if gotPath != "/addons/mail/httphandlers/download.ashx?attachid=42" {
+		t.Fatalf("path = %q", gotPath)
+	}
+}