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/cmd/oo/mails.go b/cmd/oo/mails.go
index 3b90c5a..c49f362 100644
--- a/cmd/oo/mails.go
+++ b/cmd/oo/mails.go
@@ -22,9 +22,11 @@ func init() {
 	mailsCmd.AddCommand(mailsFoldersCmd())
 	mailsCmd.AddCommand(mailsListCmd())
 	mailsCmd.AddCommand(mailsGetCmd())
+	mailsCmd.AddCommand(mailsDownloadAttachmentCmd())
 	mailsCmd.AddCommand(mailsDraftCmd())
 	mailsCmd.AddCommand(mailsAttachCmd())
 	mailsCmd.AddCommand(mailsDraftInvoiceCmd())
+	mailsCmd.AddCommand(mailsSendCmd())
 	mailsCmd.AddCommand(mailsDeleteCmd())
 }
 
@@ -131,6 +133,48 @@ func mailsGetCmd() *cobra.Command {
 	}
 }
 
+func mailsDownloadAttachmentCmd() *cobra.Command {
+	var outPath string
+	cmd := &cobra.Command{
+		Use:   "download-attachment ATTACHMENT_ID",
+		Short: "Download a mail attachment by attachment id",
+		Long: `Download a raw attachment from OnlyOffice Mail's download.ashx handler.
+
+Example:
+  oo mails download-attachment 12345 --out /tmp/attach.bin
+`,
+		Args: cobra.ExactArgs(1),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			if strings.TrimSpace(outPath) == "" {
+				return fmt.Errorf("--out is required")
+			}
+			c, err := newOO(cmd)
+			if err != nil {
+				return err
+			}
+			body, err := c.DownloadMailAttachment(cmd.Context(), args[0])
+			if err != nil {
+				return err
+			}
+			if err := writeMailAttachment(outPath, body); err != nil {
+				return err
+			}
+			if outputFormat == "json" {
+				printObject(map[string]any{
+					"attachmentId": args[0],
+					"bytes":        len(body),
+					"path":         outPath,
+				})
+				return nil
+			}
+			fmt.Printf("saved %d bytes to %s\n", len(body), outPath)
+			return nil
+		},
+	}
+	cmd.Flags().StringVar(&outPath, "out", "", "output file path")
+	return cmd
+}
+
 func mailsDraftCmd() *cobra.Command {
 	var from, to, cc, bcc, subject, body, html string
 	var id int64
@@ -297,6 +341,78 @@ func formatInvoiceCostEUR(v any) string {
 	return s
 }
 
+func writeMailAttachment(path string, body []byte) error {
+	if strings.TrimSpace(path) == "" {
+		return fmt.Errorf("attachment output path is required")
+	}
+	return os.WriteFile(path, body, 0o644)
+}
+
+func mailsSendCmd() *cobra.Command {
+	var from, to, cc, bcc, subject, body, html string
+	var id int64
+	cmd := &cobra.Command{
+		Use:   "send",
+		Short: "Send a mail message (OnlyOffice Mail)",
+		Long: `Send via PUT /api/2.0/mail/messages/send.json.
+
+  oo mails send --id 7803 --body "…"            # send referencing a draft id
+  oo mails send --to a@b.com --subject "…" --body "…"
+  oo mails send --id 7803 --to a@b.com --subject "…" --body "…" --cc x@y.com
+
+IMPORTANT: send.json does NOT copy subject/body from the referenced draft — the
+content must be in this request (--subject/--body). Cc/Bcc are omitted when empty
+(the API 400s on empty strings). The API send does not append the UI signature —
+put the chat line in --body if needed.
+`,
+		RunE: func(cmd *cobra.Command, args []string) error {
+			if to == "" && id == 0 {
+				return fmt.Errorf("--to is required (or --id of an existing draft)")
+			}
+			htmlBody := body
+			if html != "" {
+				htmlBody = html
+			}
+			if htmlBody == "" && id != 0 {
+				// The send.json endpoint does NOT copy subject/body from the
+				// referenced draft — an empty body here sends an empty message.
+				// Warn instead of silently mailing an empty email.
+				return fmt.Errorf("--body/--html is required when sending by --id (send.json needs the content in the request)")
+			}
+			if htmlBody == "" && to == "" {
+				return fmt.Errorf("--body is required for a fresh message")
+			}
+			c, err := newOO(cmd)
+			if err != nil {
+				return err
+			}
+			raw, err := c.SendMail(cmd.Context(), onlyoffice.SendMailParams{
+				ID:      id,
+				From:    from,
+				To:      to,
+				Cc:      cc,
+				Bcc:     bcc,
+				Subject: subject,
+				Body:    htmlBody,
+			})
+			if err != nil {
+				return err
+			}
+			fmt.Println(string(raw))
+			return nil
+		},
+	}
+	cmd.Flags().Int64Var(&id, "id", 0, "existing draft id to send (0 = fresh message)")
+	cmd.Flags().StringVar(&from, "from", "", "from address (default: first enabled mailbox)")
+	cmd.Flags().StringVar(&to, "to", "", "recipient (required unless --id)")
+	cmd.Flags().StringVar(&cc, "cc", "", "cc")
+	cmd.Flags().StringVar(&bcc, "bcc", "", "bcc")
+	cmd.Flags().StringVar(&subject, "subject", "", "subject")
+	cmd.Flags().StringVar(&body, "body", "", "plain text or HTML body")
+	cmd.Flags().StringVar(&html, "html", "", "HTML body (alias of --body when set)")
+	return cmd
+}
+
 func mailsDeleteCmd() *cobra.Command {
 	return &cobra.Command{
 		Use:     "delete ID [ID...]",
diff --git a/cmd/oo/mails_cli_test.go b/cmd/oo/mails_cli_test.go
new file mode 100644
index 0000000..dd69516
--- /dev/null
+++ b/cmd/oo/mails_cli_test.go
@@ -0,0 +1,28 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestWriteMailAttachment(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "attach.bin")
+	body := []byte("payload")
+	if err := writeMailAttachment(path, body); err != nil {
+		t.Fatalf("writeMailAttachment: %v", err)
+	}
+	got, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("ReadFile: %v", err)
+	}
+	if string(got) != string(body) {
+		t.Fatalf("body = %q", got)
+	}
+}
+
+func TestWriteMailAttachmentRequiresPath(t *testing.T) {
+	if err := writeMailAttachment("", []byte("x")); err == nil {
+		t.Fatal("expected error for empty path")
+	}
+}
diff --git a/cmd/oo/main.go b/cmd/oo/main.go
index 13c38fc..30ba5b6 100644
--- a/cmd/oo/main.go
+++ b/cmd/oo/main.go
@@ -13,7 +13,7 @@
 //	oo cases         list | create | delete | member-add
 //	oo crm-tasks     list | create | delete | categories
 //	oo crm           cleanup
-//	oo mails         accounts | folders | list | get | draft | attach | draft-invoice | delete
+//	oo mails         accounts | folders | list | get | download-attachment | draft | attach | draft-invoice | delete
 //	oo invoices      list | get | create | update | pdf | pdf-cleanup | status | delete | items …
 //
 // CRM association rules: docs/crm-associations.md
diff --git a/contact_indexes.go b/contact_indexes.go
new file mode 100644
index 0000000..e056476
--- /dev/null
+++ b/contact_indexes.go
@@ -0,0 +1,96 @@
+package onlyoffice
+
+import (
+	"context"
+	"fmt"
+	"sort"
+	"strconv"
+	"strings"
+)
+
+// History entities that OnlyOffice CRM actually accepts for history notes.
+// There is NO person/contact history in this API version: POST /api/2.0/crm/history.json
+// returns 400 "Value does not fall within the expected range." for entityType
+// contact/person/people/client/member. Verified against a live instance (#74).
+const (
+	HistoryEntityOpportunity = "opportunity"
+	HistoryEntityCase        = "case"
+)
+
+// IsCompany reports whether a CRM contact row is a company (vs a person).
+// The field arrives as JSON bool; be liberal about what we accept.
+func IsCompany(person map[string]any) bool {
+	b, _ := person["isCompany"].(bool)
+	return b
+}
+
+// ContactID returns the CRM id of a contact row as a plain string.
+func ContactID(row map[string]any) string {
+	return fmt.Sprint(row["id"])
+}
+
+// BuildContactEmailIndex scans all persons once and maps lowercase email →
+// contact id. Use this instead of calling FindPersonByEmail per address:
+// the index is O(N) over the whole CRM, the per-address lookup is O(N×M).
+func (c *Client) BuildContactEmailIndex(ctx context.Context) (map[string]string, error) {
+	all, err := c.ListAllContacts(ctx)
+	if err != nil {
+		return nil, err
+	}
+	index := make(map[string]string, len(all)*2)
+	for _, person := range all {
+		if IsCompany(person) {
+			continue
+		}
+		id := ContactID(person)
+		for _, row := range ContactInfoRows(person) {
+			if NormalizeContactInfoType(fmt.Sprint(row["infoType"])) != "email" {
+				continue
+			}
+			email := strings.ToLower(strings.TrimSpace(fmt.Sprint(row["data"])))
+			if email != "" && email != "<nil>" {
+				index[email] = id
+			}
+		}
+	}
+	return index, nil
+}
+
+// BuildPersonOpportunityIndex maps every opportunity member's contact id to a
+// deterministic representative opportunity: the one with the lowest numeric id.
+// OnlyOffice has no person-level history, so notes for a person go on their
+// deal — this index answers "which deal" in one pass.
+func (c *Client) BuildPersonOpportunityIndex(ctx context.Context) (map[string]string, error) {
+	opps, err := c.ListAllOpportunities(ctx)
+	if err != nil {
+		return nil, err
+	}
+	index := map[string]string{}
+	for _, opp := range opps {
+		oppID := ContactID(opp)
+		for _, member := range OpportunityMembers(opp) {
+			pid := ContactID(member)
+			if cur, ok := index[pid]; !ok || NumericIDLess(oppID, cur) {
+				index[pid] = oppID
+			}
+		}
+	}
+	return index, nil
+}
+
+// NumericIDLess compares two string ids numerically when possible, falling
+// back to lexicographic order so results stay deterministic either way.
+func NumericIDLess(a, b string) bool {
+	na, errA := strconv.Atoi(strings.TrimSpace(a))
+	nb, errB := strconv.Atoi(strings.TrimSpace(b))
+	if errA == nil && errB == nil && na != nb {
+		return na < nb
+	}
+	return a < b
+}
+
+// SortIDs orders id strings deterministically (numeric first, then lexical).
+func SortIDs(ids []string) {
+	sort.Strings(ids)
+	sort.SliceStable(ids, func(i, j int) bool { return NumericIDLess(ids[i], ids[j]) })
+}
diff --git a/contact_indexes_test.go b/contact_indexes_test.go
new file mode 100644
index 0000000..50677a9
--- /dev/null
+++ b/contact_indexes_test.go
@@ -0,0 +1,65 @@
+package onlyoffice
+
+import "testing"
+
+func TestIsCompany(t *testing.T) {
+	if IsCompany(map[string]any{"isCompany": true}) != true {
+		t.Fatal("true row not detected")
+	}
+	if IsCompany(map[string]any{"isCompany": false}) {
+		t.Fatal("false row detected as company")
+	}
+	if IsCompany(map[string]any{}) {
+		t.Fatal("missing field detected as company")
+	}
+	if IsCompany(nil) {
+		t.Fatal("nil row detected as company")
+	}
+}
+
+func TestNumericIDLess(t *testing.T) {
+	cases := []struct {
+		a, b string
+		want bool
+	}{
+		{"9", "10", true},
+		{"1747", "1748", true},
+		{"abc", "abd", true},
+		{"10", "9", false},
+		{" 12 ", "13", true},
+		{"x1", "2", false}, // non-numeric falls back lexical: "x1" > "2"
+	}
+	for _, c := range cases {
+		if got := NumericIDLess(c.a, c.b); got != c.want {
+			t.Errorf("NumericIDLess(%q,%q)=%v want %v", c.a, c.b, got, c.want)
+		}
+	}
+}
+
+func TestSortIDs(t *testing.T) {
+	ids := []string{"20", "3", "100", "1"}
+	SortIDs(ids)
+	want := "1 3 20 100"
+	got := ""
+	for i, id := range ids {
+		if i > 0 {
+			got += " "
+		}
+		got += id
+	}
+	if got != want {
+		t.Fatalf("SortIDs=%q want %q", got, want)
+	}
+}
+
+func TestContactID(t *testing.T) {
+	if ContactID(map[string]any{"id": float64(42)}) != "42" {
+		t.Fatal("numeric id formatting broken")
+	}
+}
+
+func TestHistoryEntityConstants(t *testing.T) {
+	if HistoryEntityOpportunity != "opportunity" || HistoryEntityCase != "case" {
+		t.Fatal("history entity whitelist drifted from live-verified values")
+	}
+}
diff --git a/mails.go b/mails.go
index 5689c6c..d15f821 100644
--- a/mails.go
+++ b/mails.go
@@ -7,6 +7,8 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"io"
+	"net/http"
 	"net/mail"
 	"net/url"
 	"strconv"
@@ -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) {
@@ -159,6 +193,49 @@ func (c *Client) SaveMailDraft(ctx context.Context, p SaveMailDraftParams) (map[
 	return c.putJSONObject(ctx, "/api/2.0/mail/drafts/save", body)
 }
 
+// SendMailParams describes a message to send via PUT /api/2.0/mail/messages/send.
+// ID refers to an existing draft/message id; From falls back to the first enabled
+// mailbox. Cc/Bcc are omitted when empty (the API 400s on empty strings). Chat
+// line goes into Body (API send does not append the UI signature).
+type SendMailParams struct {
+	ID      int64
+	From    string
+	To      string
+	Cc      string
+	Bcc     string
+	Subject string
+	Body    string // HTML
+}
+
+// SendMail sends an existing draft (or a fresh message) via the OnlyOffice Mail
+// send endpoint. Returns the raw send response.
+func (c *Client) SendMail(ctx context.Context, p SendMailParams) (json.RawMessage, error) {
+	if strings.TrimSpace(p.To) == "" {
+		return nil, fmt.Errorf("SendMail: to is required")
+	}
+	if strings.TrimSpace(p.From) == "" {
+		from, err := c.defaultMailFrom(ctx)
+		if err != nil {
+			return nil, err
+		}
+		p.From = from
+	}
+	body := map[string]any{
+		"id":      p.ID,
+		"from":    p.From,
+		"to":      p.To,
+		"subject": p.Subject,
+		"body":    p.Body,
+	}
+	if strings.TrimSpace(p.Cc) != "" {
+		body["cc"] = p.Cc
+	}
+	if strings.TrimSpace(p.Bcc) != "" {
+		body["bcc"] = p.Bcc
+	}
+	return c.putJSON(ctx, "/api/2.0/mail/messages/send.json", body)
+}
+
 func (c *Client) defaultMailFrom(ctx context.Context) (string, error) {
 	accounts, err := c.ListMailAccounts(ctx)
 	if err != nil {
diff --git a/mails_test.go b/mails_test.go
index be81255..bf9a35c 100644
--- a/mails_test.go
+++ b/mails_test.go
@@ -1,8 +1,14 @@
 package onlyoffice
 
 import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"net/http/cookiejar"
+	"net/http/httptest"
 	"strings"
 	"testing"
+	"time"
 )
 
 func TestResolveMailFolder(t *testing.T) {
@@ -44,9 +50,9 @@ func TestMailMessagesPath(t *testing.T) {
 
 func TestParseMailAddress(t *testing.T) {
 	tests := []struct {
-		raw          string
-		wantName     string
-		wantAddress  string
+		raw         string
+		wantName    string
+		wantAddress string
 	}{
 		{`"LinkedIn Jobbenachrichtigungen" <jobalerts-noreply@linkedin.com>`, "LinkedIn Jobbenachrichtigungen", "jobalerts-noreply@linkedin.com"},
 		{`"Bitfinex" <no-reply@bitfinex.com>`, "Bitfinex", "no-reply@bitfinex.com"},
@@ -97,3 +103,112 @@ 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)
+	}
+}
+
+func TestSendMailOmitsEmptyCcBcc(t *testing.T) {
+	var gotBody map[string]any
+	var gotPath string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		switch r.URL.Path {
+		case "/api/2.0/authentication.json":
+			w.Header().Set("Content-Type", "application/json")
+			_, _ = w.Write([]byte(`{"response":{"token":"tok","expires":"2099-01-01T00:00:00.0000000+00:00"}}`))
+		case "/api/2.0/mail/messages/send.json":
+			gotPath = r.URL.Path
+			dec := json.NewDecoder(r.Body)
+			_ = dec.Decode(&gotBody)
+			w.Header().Set("Content-Type", "application/json")
+			_, _ = w.Write([]byte(`{"response":{"id":1}}`))
+		default:
+			http.NotFound(w, r)
+		}
+	}))
+	defer srv.Close()
+
+	c := NewClient(Credentials{Url: srv.URL, User: "u", Password: "p"})
+	ctx := context.Background()
+	raw, err := c.SendMail(ctx, SendMailParams{
+		ID:      99,
+		From:    "me@x.com",
+		To:      "a@b.com",
+		Subject: "hi",
+		Body:    "<p>hello</p>",
+	})
+	if err != nil {
+		t.Fatalf("SendMail: %v", err)
+	}
+	if gotPath != "/api/2.0/mail/messages/send.json" {
+		t.Fatalf("path = %q", gotPath)
+	}
+	if _, hasCC := gotBody["cc"]; hasCC {
+		t.Fatalf("empty cc should be omitted: %v", gotBody)
+	}
+	if _, hasBcc := gotBody["bcc"]; hasBcc {
+		t.Fatalf("empty bcc should be omitted: %v", gotBody)
+	}
+	if gotBody["to"] != "a@b.com" {
+		t.Fatalf("to = %v", gotBody["to"])
+	}
+	if gotBody["id"] != float64(99) {
+		t.Fatalf("id = %v", gotBody["id"])
+	}
+	if !strings.Contains(string(raw), `"id"`) {
+		t.Fatalf("raw = %s", raw)
+	}
+}