diff --git a/README.md b/README.md
index c369651..6a02388 100644
--- a/README.md
+++ b/README.md
@@ -611,6 +611,12 @@ Deletes a draft message and all its attachments from the database and disk. Only
 
 Marks a draft message as sent by setting `time_sent` to the current timestamp. Only the owner may send.
 
+**Send is the validation gate.** Drafts are a workspace: create/update accept
+incomplete messages (no recipients, no type) by design, but a draft may only
+be *sent* when it is a complete, valid fmsg message — at least one
+well-formed recipient, a `type`, and a supported `version`. An incomplete
+draft is refused with `400` listing every problem.
+
 For a reply (a draft with `pid`), the route first verifies that every remote
 recipient domain can actually accept it: per the fmsg spec a host rejects a
 reply whose parent it has not stored (response code 6), so if the parent was
@@ -631,6 +637,7 @@ recipients are unaffected.
 
 | Status | Condition |
 | ------ | --------- |
+| `400`  | Message is not sendable (no recipients, invalid recipient address, no type, unsupported version) |
 | `403`  | Not the owner |
 | `404`  | Message not found |
 | `409`  | Message already sent |
diff --git a/internal/handlers/messages.go b/internal/handlers/messages.go
index f32e789..a15c32a 100644
--- a/internal/handlers/messages.go
+++ b/internal/handlers/messages.go
@@ -884,6 +884,13 @@ func (h *MessageHandler) Send(c *gin.Context) {
 		return
 	}
 
+	// Drafts are a workspace — create/update accept incomplete messages by
+	// design. Send is the gate: only a complete, valid message may leave.
+	if problems := sendableProblems(existing); len(problems) > 0 {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "message is not sendable: " + strings.Join(problems, "; ")})
+		return
+	}
+
 	// A reply can only be accepted by hosts that store its parent (SPEC
 	// §10.3, reject code 6). Refuse now — with the reason — when a remote
 	// recipient domain can never accept it, rather than letting the reply
@@ -1583,6 +1590,30 @@ func (h *MessageHandler) extractShortText(dataPath, mimeType string) string {
 	return string(buf)
 }
 
+// sendableProblems lists everything preventing a draft from being sent as a
+// valid fmsg message. Create/update deliberately accept incomplete drafts
+// (a draft is a workspace); completeness is enforced only here, at send.
+// Address *form* is still validated on create/update, but old rows predate
+// that, so it is re-checked defensively.
+func sendableProblems(m *models.Message) []string {
+	var problems []string
+	if m.Version != 1 {
+		problems = append(problems, fmt.Sprintf("unsupported version %d", m.Version))
+	}
+	if len(m.To) == 0 {
+		problems = append(problems, "no recipients")
+	}
+	for _, addr := range m.To {
+		if !middleware.IsValidAddr(addr) {
+			problems = append(problems, fmt.Sprintf("invalid recipient address %q", addr))
+		}
+	}
+	if m.Type == "" {
+		problems = append(problems, "no type")
+	}
+	return problems
+}
+
 // validateAddresses returns an error if the from address or any to address is
 // not a valid "@user@domain" address. (add_to recipients are validated by the
 // add-to route, not on create/update.)
diff --git a/internal/handlers/messages_test.go b/internal/handlers/messages_test.go
index a7ac464..1c5b7e1 100644
--- a/internal/handlers/messages_test.go
+++ b/internal/handlers/messages_test.go
@@ -334,3 +334,44 @@ func TestRemoteRecipientDomains(t *testing.T) {
 		t.Fatalf("domains = %v", got)
 	}
 }
+
+func TestSendableProblems(t *testing.T) {
+	valid := func() *models.Message {
+		return &models.Message{
+			Version: 1,
+			To:      []string{"@bob@remote.example"},
+			Type:    "text/markdown",
+		}
+	}
+
+	if got := sendableProblems(valid()); len(got) != 0 {
+		t.Fatalf("valid draft flagged: %v", got)
+	}
+
+	tests := []struct {
+		name   string
+		mutate func(*models.Message)
+		want   string
+	}{
+		{"no recipients", func(m *models.Message) { m.To = nil }, "no recipients"},
+		{"invalid recipient", func(m *models.Message) { m.To = []string{"bob@remote.example"} }, "invalid recipient"},
+		{"no type", func(m *models.Message) { m.Type = "" }, "no type"},
+		{"bad version", func(m *models.Message) { m.Version = 0 }, "unsupported version"},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			m := valid()
+			tc.mutate(m)
+			got := sendableProblems(m)
+			if len(got) != 1 || !strings.Contains(got[0], tc.want) {
+				t.Fatalf("problems = %v, want one containing %q", got, tc.want)
+			}
+		})
+	}
+
+	// Everything wrong at once: every problem reported, not just the first.
+	m := &models.Message{Version: 0}
+	if got := sendableProblems(m); len(got) != 3 {
+		t.Fatalf("problems = %v, want 3", got)
+	}
+}