diff --git a/cmd/oo/mails.go b/cmd/oo/mails.go
index d6d5b4d..c49f362 100644
--- a/cmd/oo/mails.go
+++ b/cmd/oo/mails.go
@@ -26,6 +26,7 @@ func init() {
 	mailsCmd.AddCommand(mailsDraftCmd())
 	mailsCmd.AddCommand(mailsAttachCmd())
 	mailsCmd.AddCommand(mailsDraftInvoiceCmd())
+	mailsCmd.AddCommand(mailsSendCmd())
 	mailsCmd.AddCommand(mailsDeleteCmd())
 }
 
@@ -347,6 +348,71 @@ func writeMailAttachment(path string, body []byte) error {
 	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/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/files.go b/files.go
index 794821c..c0d0cdd 100644
--- a/files.go
+++ b/files.go
@@ -319,8 +319,18 @@ func (c *Client) resolveAPIURL(ref string) string {
 	if ref == "" {
 		return ref
 	}
-	if strings.HasPrefix(ref, "http://") || strings.HasPrefix(ref, "https://") {
-		return ref
+	// Rewrite any host to the configured API base so downloads stay on the
+	// internal network and keep the Authorization header (no cross-host
+	// redirect that would strip it). Scheme-relative URLs are handled too.
+	if strings.HasPrefix(ref, "//") {
+		ref = "http:" + ref
+	}
+	if u, err := url.Parse(ref); err == nil && u.IsAbs() {
+		if base, err2 := url.Parse(c.baseURL()); err2 == nil {
+			u.Scheme = base.Scheme
+			u.Host = base.Host
+			return u.String()
+		}
 	}
 	base := c.baseURL()
 	if strings.HasPrefix(ref, "/") {
diff --git a/files_webdav.go b/files_webdav.go
new file mode 100644
index 0000000..f3fcafd
--- /dev/null
+++ b/files_webdav.go
@@ -0,0 +1,421 @@
+package onlyoffice
+
+// WebDAV-oriented Files operations. These expose the Documents module through
+// value types and cover everything needed to back a filesystem mapping:
+// listing (including the virtual @root sections), folder/file CRUD, move/copy,
+// and streaming upload/download. They are intentionally small and dependency
+// free (only net/http), so callers are not forced to import heavier parts of
+// the library.
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"mime/multipart"
+	"net/http"
+	"net/url"
+	"strconv"
+	"strings"
+	"time"
+)
+
+// DavFolder is a folder row from the Files module.
+type DavFolder struct {
+	ID           string
+	Title        string
+	ParentID     string
+	RootType     int // 1=Common, 3=Trash, 5=My, 6=Share, 8=Projects, ...
+	FilesCount   int
+	FoldersCount int
+	Access       int
+	Shared       bool
+	Updated      string
+}
+
+// DavFile is a file row from the Files module.
+type DavFile struct {
+	ID      string
+	Title   string
+	Size    int64
+	Updated string
+	ViewURL string
+}
+
+// DavListing is the contents of one folder.
+type DavListing struct {
+	Current DavFolder
+	Files   []DavFile
+	Folders []DavFolder
+}
+
+// ListDavFolder returns the contents of a folder by id, which may be a
+// symbolic root such as "@my". For "@root" use ListDavSections.
+func (c *Client) ListDavFolder(ctx context.Context, id string) (*DavListing, error) {
+	raw, err := c.getJSON(ctx, "/api/2.0/files/"+url.PathEscape(id))
+	if err != nil {
+		return nil, err
+	}
+	resp, err := responseField(raw, "response")
+	if err != nil {
+		return nil, err
+	}
+	// @root returns an array with a single blob; a normal folder returns an
+	// object. Normalize both.
+	if len(resp) > 0 && resp[0] == '[' {
+		var arr []*DavListing
+		if err := json.Unmarshal(resp, &arr); err != nil {
+			return nil, err
+		}
+		if len(arr) == 0 {
+			return &DavListing{}, nil
+		}
+		return arr[0], nil
+	}
+	var l DavListing
+	if err := json.Unmarshal(resp, &l); err != nil {
+		return nil, err
+	}
+	return &l, nil
+}
+
+// ListDavSections returns the virtual top-level sections shown by @root
+// ("In projects", "My documents", "Shared with me", "Common", "Favorites",
+// "Recent", "Trash"). Each is the `current` folder of one @root element.
+func (c *Client) ListDavSections(ctx context.Context) ([]DavFolder, error) {
+	raw, err := c.getJSON(ctx, "/api/2.0/files/@root")
+	if err != nil {
+		return nil, err
+	}
+	resp, err := responseField(raw, "response")
+	if err != nil {
+		return nil, err
+	}
+	var arr []struct {
+		Current DavFolder `json:"current"`
+	}
+	if err := json.Unmarshal(resp, &arr); err != nil {
+		// Tolerate a non-array (single listing) response.
+		var single DavListing
+		if err2 := json.Unmarshal(resp, &single); err2 != nil {
+			return nil, err
+		}
+		return []DavFolder{single.Current}, nil
+	}
+	sections := make([]DavFolder, 0, len(arr))
+	for i := range arr {
+		sections = append(sections, arr[i].Current)
+	}
+	return sections, nil
+}
+
+// CreateDavFolder creates a folder titled title inside parentID.
+func (c *Client) CreateDavFolder(ctx context.Context, parentID, title string) (*DavFolder, error) {
+	raw, err := c.postJSON(ctx, "/api/2.0/files/folder/"+url.PathEscape(parentID),
+		map[string]string{"title": title})
+	if err != nil {
+		return nil, err
+	}
+	var env struct {
+		Response *DavFolder `json:"response"`
+	}
+	if err := json.Unmarshal(raw, &env); err != nil {
+		return nil, err
+	}
+	if env.Response == nil {
+		return nil, fmt.Errorf("onlyoffice: empty create-folder response")
+	}
+	return env.Response, nil
+}
+
+// RenameDavFolder renames a folder.
+func (c *Client) RenameDavFolder(ctx context.Context, id, title string) error {
+	_, err := c.putJSON(ctx, "/api/2.0/files/folder/"+url.PathEscape(id),
+		map[string]string{"title": title})
+	return err
+}
+
+// RenameDavFile renames a file (title includes the extension).
+func (c *Client) RenameDavFile(ctx context.Context, id, title string) error {
+	_, err := c.putJSON(ctx, "/api/2.0/files/file/"+url.PathEscape(id),
+		map[string]string{"title": title})
+	return err
+}
+
+// MoveDavItems moves the given folders and/or files into destFolderID.
+func (c *Client) MoveDavItems(ctx context.Context, folderIDs, fileIDs []string, destFolderID string) error {
+	_, err := c.putJSON(ctx, "/api/2.0/files/fileops/move", map[string]any{
+		"folderIds":    nums(folderIDs),
+		"fileIds":      nums(fileIDs),
+		"destFolderId": num(destFolderID),
+		"resolveType":  "Skip",
+		"holdResult":   true,
+	})
+	return err
+}
+
+// CopyDavItems copies the given folders and/or files into destFolderID.
+func (c *Client) CopyDavItems(ctx context.Context, folderIDs, fileIDs []string, destFolderID string) error {
+	_, err := c.putJSON(ctx, "/api/2.0/files/fileops/copy", map[string]any{
+		"folderIds":           nums(folderIDs),
+		"fileIds":             nums(fileIDs),
+		"destFolderId":        num(destFolderID),
+		"conflictResolveType": "Skip",
+		"deleteAfter":         true,
+	})
+	return err
+}
+
+// DeleteDavItems deletes the given folders and/or files.
+func (c *Client) DeleteDavItems(ctx context.Context, folderIDs, fileIDs []string) error {
+	body := map[string]any{"DeleteAfter": true, "Immediately": false}
+	for _, id := range folderIDs {
+		if _, err := c.deleteJSON(ctx, "/api/2.0/files/folder/"+url.PathEscape(id), body); err != nil {
+			return err
+		}
+	}
+	for _, id := range fileIDs {
+		if _, err := c.deleteJSON(ctx, "/api/2.0/files/file/"+url.PathEscape(id), body); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// UploadDavFile uploads src (fileName) into folderID, streaming from src.
+func (c *Client) UploadDavFile(ctx context.Context, folderID, fileName string, src io.Reader) (*DavFile, error) {
+	raw, err := c.uploadReader(ctx, "/api/2.0/files/"+url.PathEscape(folderID)+"/upload", "file", fileName, src)
+	if err != nil {
+		return nil, err
+	}
+	var env struct {
+		Response *DavFile `json:"response"`
+	}
+	if err := json.Unmarshal(raw, &env); err != nil {
+		return nil, err
+	}
+	if env.Response == nil {
+		return nil, fmt.Errorf("onlyoffice: empty upload response")
+	}
+	return env.Response, nil
+}
+
+// DownloadDavFile streams the file identified by id to w, returning bytes copied.
+func (c *Client) DownloadDavFile(ctx context.Context, id string, w io.Writer) (int64, error) {
+	file, err := c.GetFile(ctx, id)
+	if err != nil {
+		return 0, err
+	}
+	if file.ViewURL == nil || *file.ViewURL == "" {
+		return 0, fmt.Errorf("onlyoffice: file %s has no viewUrl", id)
+	}
+	u := c.resolveAPIURL(*file.ViewURL)
+	auth, err := c.authHeader()
+	if err != nil {
+		return 0, err
+	}
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, u, nil)
+	if err != nil {
+		return 0, err
+	}
+	req.Header.Set("Authorization", auth)
+	resp, err := c.client.Do(req)
+	if err != nil {
+		return 0, err
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode >= 400 {
+		return 0, fmt.Errorf("onlyoffice: download: %d", resp.StatusCode)
+	}
+	return io.Copy(w, resp.Body)
+}
+
+// --- internal helpers -------------------------------------------------------
+
+// deleteJSON performs an authenticated DELETE with an optional JSON body.
+func (c *Client) deleteJSON(ctx context.Context, path string, body any) (json.RawMessage, error) {
+	var reader io.Reader
+	if body != nil {
+		buf, err := json.Marshal(body)
+		if err != nil {
+			return nil, err
+		}
+		reader = bytes.NewReader(buf)
+	}
+	auth, err := c.authHeader()
+	if err != nil {
+		return nil, err
+	}
+	req, err := http.NewRequestWithContext(ctx, http.MethodDelete, c.baseURL()+path, reader)
+	if err != nil {
+		return nil, err
+	}
+	req.Header.Set("Authorization", auth)
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Accept", "application/json")
+	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("DELETE %s: %d %s", path, resp.StatusCode, truncate(string(raw), 400))
+	}
+	return raw, nil
+}
+
+// uploadReader uploads a stream to path under the given form field name.
+func (c *Client) uploadReader(ctx context.Context, path, fieldName, fileName string, src io.Reader) (json.RawMessage, error) {
+	var buf bytes.Buffer
+	mw := multipart.NewWriter(&buf)
+	part, err := mw.CreateFormFile(fieldName, fileName)
+	if err != nil {
+		return nil, err
+	}
+	if _, err := io.Copy(part, src); err != nil {
+		return nil, err
+	}
+	if err := mw.Close(); err != nil {
+		return nil, err
+	}
+	auth, err := c.authHeader()
+	if err != nil {
+		return nil, err
+	}
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, c.baseURL()+path, &buf)
+	if err != nil {
+		return nil, err
+	}
+	req.Header.Set("Authorization", auth)
+	req.Header.Set("Content-Type", mw.FormDataContentType())
+	req.Header.Set("Accept", "application/json")
+	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("upload %s: %d %s", path, resp.StatusCode, truncate(string(raw), 400))
+	}
+	return raw, nil
+}
+
+func nums(ids []string) []json.Number {
+	out := make([]json.Number, 0, len(ids))
+	for _, id := range ids {
+		if _, err := strconv.Atoi(id); err == nil {
+			out = append(out, json.Number(id))
+		}
+	}
+	return out
+}
+
+func num(id string) any {
+	if _, err := strconv.Atoi(id); err == nil {
+		return json.Number(id)
+	}
+	return id
+}
+
+// UnmarshalJSON decodes a folder from the portal envelope, including fields
+// that the base FolderEntry omits (parentId, rootFolderType, access, ...).
+func (f *DavFolder) UnmarshalJSON(b []byte) error {
+	var raw struct {
+		ID           *json.Number `json:"id"`
+		Title        *string      `json:"title"`
+		ParentID     *json.Number `json:"parentId"`
+		RootType     *int         `json:"rootFolderType"`
+		FilesCount   *int         `json:"filesCount"`
+		FoldersCount *int         `json:"foldersCount"`
+		Access       *int         `json:"access"`
+		Shared       *bool        `json:"shared"`
+		Updated      *string      `json:"updated"`
+	}
+	if err := json.Unmarshal(b, &raw); err != nil {
+		return err
+	}
+	if raw.ID != nil {
+		f.ID = raw.ID.String()
+	}
+	if raw.Title != nil {
+		f.Title = *raw.Title
+	}
+	if raw.ParentID != nil {
+		f.ParentID = raw.ParentID.String()
+	}
+	if raw.RootType != nil {
+		f.RootType = *raw.RootType
+	}
+	if raw.FilesCount != nil {
+		f.FilesCount = *raw.FilesCount
+	}
+	if raw.FoldersCount != nil {
+		f.FoldersCount = *raw.FoldersCount
+	}
+	if raw.Access != nil {
+		f.Access = *raw.Access
+	}
+	if raw.Shared != nil {
+		f.Shared = *raw.Shared
+	}
+	if raw.Updated != nil {
+		f.Updated = *raw.Updated
+	}
+	return nil
+}
+
+// UnmarshalJSON decodes a file row, capturing size and timestamps.
+func (f *DavFile) UnmarshalJSON(b []byte) error {
+	var raw struct {
+		ID        *json.Number `json:"id"`
+		Title     *string      `json:"title"`
+		PureSize  *int64       `json:"pureContentLength"`
+		SizeStr   *string      `json:"contentLength"`
+		Updated   *string      `json:"updated"`
+		ViewURL   *string      `json:"viewUrl"`
+	}
+	if err := json.Unmarshal(b, &raw); err != nil {
+		return err
+	}
+	if raw.ID != nil {
+		f.ID = raw.ID.String()
+	}
+	if raw.Title != nil {
+		f.Title = *raw.Title
+	}
+	if raw.PureSize != nil {
+		f.Size = *raw.PureSize
+	} else if raw.SizeStr != nil {
+		if n, err := strconv.ParseInt(strings.Fields(*raw.SizeStr)[0], 10, 64); err == nil {
+			f.Size = n
+		}
+	}
+	if raw.Updated != nil {
+		f.Updated = *raw.Updated
+	}
+	if raw.ViewURL != nil {
+		f.ViewURL = *raw.ViewURL
+	}
+	return nil
+}
+
+// ModTime parses the folder's updated timestamp.
+func (f *DavFolder) ModTime() time.Time {
+	t, _ := time.Parse("2006-01-02T15:04:05.0000000-07:00", f.Updated)
+	return t
+}
+
+// ModTime parses the file's updated timestamp.
+func (f *DavFile) ModTime() time.Time {
+	t, _ := time.Parse("2006-01-02T15:04:05.0000000-07:00", f.Updated)
+	return t
+}
diff --git a/mails.go b/mails.go
index 491f725..d15f821 100644
--- a/mails.go
+++ b/mails.go
@@ -8,8 +8,8 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
-	"net/mail"
 	"net/http"
+	"net/mail"
 	"net/url"
 	"strconv"
 	"strings"
@@ -193,6 +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 86b2374..bf9a35c 100644
--- a/mails_test.go
+++ b/mails_test.go
@@ -2,6 +2,7 @@ package onlyoffice
 
 import (
 	"context"
+	"encoding/json"
 	"net/http"
 	"net/http/cookiejar"
 	"net/http/httptest"
@@ -49,9 +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"},
@@ -159,3 +160,55 @@ func TestDownloadMailAttachmentUsesAuthCookie(t *testing.T) {
 		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)
+	}
+}