diff --git a/packages/server/src/routes/dm.ts b/packages/server/src/routes/dm.ts
index 5d925999..9f8acb0e 100644
--- a/packages/server/src/routes/dm.ts
+++ b/packages/server/src/routes/dm.ts
@@ -3,7 +3,7 @@ import { eq, and, or, desc, lt, inArray, isNull, sql } from 'drizzle-orm';
 import { getDb, schema } from '../db/index.js';
 import { authenticate } from '../utils/auth.js';
 import { generateSnowflake } from '../utils/snowflake.js';
-import { isDmMember } from '../utils/permissions.js';
+import { isDmMember, isDeadOneOnOne } from '../utils/permissions.js';
 import { connectionManager } from '../ws/handler.js';
 import {
   MAX_MESSAGE_LENGTH,
@@ -2456,6 +2456,10 @@ export async function dmRoutes(app: FastifyInstance): Promise<void> {
       return reply.code(403).send({ error: 'You are not a member of this DM channel', statusCode: 403 });
     }
 
+    if (isDeadOneOnOne(id, request.userId)) {
+      return reply.code(403).send({ error: "This user's account was deleted", code: 'recipient_deleted', statusCode: 403 });
+    }
+
     const hasContent = content && typeof content === 'string' && content.trim().length > 0;
     const hasAttachments = attachmentIds && attachmentIds.length > 0;
 
@@ -2548,6 +2552,10 @@ export async function dmRoutes(app: FastifyInstance): Promise<void> {
       return reply.code(403).send({ error: 'You can only edit your own messages', statusCode: 403 });
     }
 
+    if (isDeadOneOnOne(msg.dmChannelId, request.userId)) {
+      return reply.code(403).send({ error: "This user's account was deleted", code: 'recipient_deleted', statusCode: 403 });
+    }
+
     const now = Date.now();
     db.update(schema.dmMessages)
       .set({ content: content.trim(), editedAt: now })
@@ -2600,6 +2608,10 @@ export async function dmRoutes(app: FastifyInstance): Promise<void> {
       return reply.code(403).send({ error: 'You can only delete your own messages', statusCode: 403 });
     }
 
+    if (isDeadOneOnOne(msg.dmChannelId, request.userId)) {
+      return reply.code(403).send({ error: "This user's account was deleted", code: 'recipient_deleted', statusCode: 403 });
+    }
+
     // Collect attachment filenames before deleting
     const attachmentRows = db.select({ filename: schema.attachments.filename })
       .from(schema.attachments)
diff --git a/packages/server/src/utils/permissions.ts b/packages/server/src/utils/permissions.ts
index 32495387..c2de7850 100644
--- a/packages/server/src/utils/permissions.ts
+++ b/packages/server/src/utils/permissions.ts
@@ -1,4 +1,4 @@
-import { eq, and } from 'drizzle-orm';
+import { eq, and, sql } from 'drizzle-orm';
 import { getDb, schema } from '../db/index.js';
 import {
   PermissionBits,
@@ -261,6 +261,28 @@ export function isDmMember(dmChannelId: string, userId: string): boolean {
   return member !== undefined;
 }
 
+/**
+ * True when a DM is a 1-on-1 (ownerId NULL) whose only other participant(s)
+ * are tombstoned (isDeleted=1). Used to make a Deleted-User thread read-only:
+ * no message create/edit/delete, so we never enqueue doomed/mis-directed relays.
+ */
+export function isDeadOneOnOne(dmChannelId: string, requesterId: string): boolean {
+  const db = getDb();
+  const channel = db.select({ ownerId: schema.dmChannels.ownerId })
+    .from(schema.dmChannels).where(eq(schema.dmChannels.id, dmChannelId)).get();
+  if (!channel || channel.ownerId !== null) return false; // groups are never a dead 1-on-1
+  const others = db.select({ isDeleted: schema.users.isDeleted })
+    .from(schema.dmMembers)
+    .innerJoin(schema.users, eq(schema.dmMembers.userId, schema.users.id))
+    .where(and(
+      eq(schema.dmMembers.dmChannelId, dmChannelId),
+      sql`${schema.dmMembers.userId} != ${requesterId}`,
+    ))
+    .all();
+  if (others.length === 0) return false;
+  return others.every(o => o.isDeleted === 1);
+}
+
 export function isBanned(spaceId: string, userId: string): boolean {
   const db = getDb();
   const ban = db.select().from(schema.bans)
diff --git a/packages/server/test/federation-identity-deletion.test.ts b/packages/server/test/federation-identity-deletion.test.ts
index c184f3d0..957bf6c4 100644
--- a/packages/server/test/federation-identity-deletion.test.ts
+++ b/packages/server/test/federation-identity-deletion.test.ts
@@ -402,6 +402,55 @@ describe('Federation identity deletion — server suite', () => {
     remote.close();
   });
 
+  it('#19 read-only: mutations on a Deleted-User 1-on-1 are rejected 403 recipient_deleted', async () => {
+    const fx = await setupFullDeletionFixture('t19');
+
+    // Survivor authors a message BEFORE the deletion so there is a message THEY own
+    // to edit/delete — this ensures the read-only guard (not the ownership 403) is
+    // what's exercised on the PATCH/DELETE paths, which resolve via msg.dmChannelId.
+    const pre = await fetch(`${harness.remote.origin}/api/dm/${fx.dmChannelId}/messages`, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json', Authorization: `Bearer ${fx.observerOnRemote.token}` },
+      body: JSON.stringify({ content: 'before deletion' }),
+    });
+    expect(pre.status).toBe(201);
+    const { id: messageId } = await pre.json() as { id: string };
+
+    // Tombstone the remote (federated) user via soft delete
+    const del = await fetch(`${harness.home.origin}/api/users/@me/federation-identity/delete`, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json', Authorization: `Bearer ${fx.homeUser.token}` },
+      body: JSON.stringify({ origins: [harness.remote.origin], mode: 'soft' }),
+    });
+    expect(del.status).toBe(200);
+
+    // Survivor (observerOnRemote) POST -> 403 recipient_deleted
+    const post = await fetch(`${harness.remote.origin}/api/dm/${fx.dmChannelId}/messages`, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json', Authorization: `Bearer ${fx.observerOnRemote.token}` },
+      body: JSON.stringify({ content: 'still there?' }),
+    });
+    expect(post.status).toBe(403);
+    expect((await post.json()).code).toBe('recipient_deleted');
+
+    // PATCH the survivor's own message (resolves channel via msg.dmChannelId) -> 403
+    const patch = await fetch(`${harness.remote.origin}/api/dm/messages/${messageId}`, {
+      method: 'PATCH',
+      headers: { 'Content-Type': 'application/json', Authorization: `Bearer ${fx.observerOnRemote.token}` },
+      body: JSON.stringify({ content: 'edit' }),
+    });
+    expect(patch.status).toBe(403);
+    expect((await patch.json()).code).toBe('recipient_deleted');
+
+    // DELETE the survivor's own message (resolves channel via msg.dmChannelId) -> 403
+    const remove = await fetch(`${harness.remote.origin}/api/dm/messages/${messageId}`, {
+      method: 'DELETE',
+      headers: { Authorization: `Bearer ${fx.observerOnRemote.token}` },
+    });
+    expect(remove.status).toBe(403);
+    expect((await remove.json()).code).toBe('recipient_deleted');
+  });
+
   it('#7 owned-spaces 409: ownership prevents deletion, registry preserved', async () => {
     const { createFederatedUser } = await import('./helpers/testUsers.js');
     const { openInspector } = await import('./helpers/dbInspect.js');