diff --git a/keyserver/src/fetchers/thread-fetchers.js b/keyserver/src/fetchers/thread-fetchers.js
--- a/keyserver/src/fetchers/thread-fetchers.js
+++ b/keyserver/src/fetchers/thread-fetchers.js
@@ -71,7 +71,7 @@
   return SQL`WHERE `.append(clause);
 }
 
-type FetchServerThreadInfosResult = {
+export type FetchServerThreadInfosResult = {
   +threadInfos: { +[id: string]: ServerThreadInfo },
 };
 
diff --git a/keyserver/src/push/send.js b/keyserver/src/push/send.js
--- a/keyserver/src/push/send.js
+++ b/keyserver/src/push/send.js
@@ -10,6 +10,7 @@
 import t from 'tcomb';
 import uuidv4 from 'uuid/v4.js';
 
+import genesis from 'lib/facts/genesis.js';
 import { oldValidUsernameRegex } from 'lib/shared/account-utils.js';
 import { isUserMentioned } from 'lib/shared/mention-utils.js';
 import {
@@ -69,7 +70,10 @@
 import { dbQuery, SQL, mergeOrConditions } from '../database/database.js';
 import type { CollapsableNotifInfo } from '../fetchers/message-fetchers.js';
 import { fetchCollapsableNotifs } from '../fetchers/message-fetchers.js';
-import { fetchServerThreadInfos } from '../fetchers/thread-fetchers.js';
+import {
+  fetchServerThreadInfos,
+  type FetchServerThreadInfosResult,
+} from '../fetchers/thread-fetchers.js';
 import { fetchUserInfos } from '../fetchers/user-fetchers.js';
 import type { Viewer } from '../session/viewer.js';
 import { getENSNames } from '../utils/ens-cache.js';
@@ -397,6 +401,110 @@
   await saveNotifResults(deliveryResults, notifications, true);
 }
 
+const baseParentTraverseQuery = (threadID: string) => SQL`
+  WITH RECURSIVE thread_tree AS (
+    SELECT id, parent_thread_id
+    FROM threads
+    WHERE id = ${threadID}
+    UNION ALL
+    SELECT t.id, t.parent_thread_id
+    FROM threads t
+    JOIN thread_tree tt ON t.id = tt.parent_thread_id
+  )
+  SELECT id FROM thread_tree
+  `;
+
+async function isThreadMentionable(
+  messageThreadID: string,
+  mentionedThreadID: string,
+  fetchThreadResult: FetchServerThreadInfosResult,
+) {
+  const threadInfos = fetchThreadResult.threadInfos;
+  if (
+    !(mentionedThreadID in threadInfos) ||
+    messageThreadID === mentionedThreadID
+  ) {
+    return false;
+  }
+
+  const messageThread = threadInfos[messageThreadID];
+  const mentionedThread = threadInfos[mentionedThreadID];
+  if (!messageThread || !mentionedThread) {
+    return false;
+  }
+  if (!messageThread.community) {
+    if (messageThread.id === genesis.id) {
+      return false;
+    }
+    if (!mentionedThread.community) {
+      return false;
+    }
+    return messageThread.id === mentionedThread.community;
+  }
+  if (messageThread.community === genesis.id) {
+    if (mentionedThread.community !== genesis.id) {
+      return false;
+    }
+    // Check if two threads share the same first child of GENESIS
+    // using the baseParentTraverseQuery which returns the path
+    // from the thread to the GENESIS community.
+    const [mentionedThreadChildren] = await dbQuery(
+      baseParentTraverseQuery(mentionedThreadID),
+    );
+    const mentionedThreadChildrenTraversePath = mentionedThreadChildren.map(
+      row => row.id.toString(),
+    );
+    const mentionedThreadGenesisFirstChildID =
+      mentionedThreadChildrenTraversePath[
+        mentionedThreadChildrenTraversePath.length - 2
+      ];
+    if (messageThread.parentThreadID === genesis.id) {
+      return mentionedThreadGenesisFirstChildID === messageThreadID;
+    }
+    const [messageThreadChildren] = await dbQuery(
+      baseParentTraverseQuery(messageThreadID),
+    );
+    const messageThreadChildrenTraversePath = messageThreadChildren.map(row =>
+      row.id.toString(),
+    );
+    const messageThreadGenesisFirstChildID =
+      messageThreadChildrenTraversePath[
+        messageThreadChildrenTraversePath.length - 2
+      ];
+    return (
+      mentionedThreadGenesisFirstChildID === messageThreadGenesisFirstChildID
+    );
+  }
+  if (!mentionedThread.community) {
+    return messageThread.community === mentionedThread.id;
+  }
+  return messageThread.community === mentionedThread.community;
+}
+
+// eslint-disable-next-line no-unused-vars
+async function getMentionableThreads(
+  messageThreadID: string,
+  mentionedThreadIDs: $ReadOnlySet<string>,
+  userID: string,
+): Promise<Set<string>> {
+  const allowedToMentionThreadIDs = new Set();
+  const threadFetchResult = await fetchServerThreadInfos({
+    threadIDs: new Set([messageThreadID, ...mentionedThreadIDs]),
+    accessibleToUserID: userID,
+  });
+  for (const mentionedThreadID of mentionedThreadIDs) {
+    const isThreadMentionableResult = await isThreadMentionable(
+      messageThreadID,
+      mentionedThreadID,
+      threadFetchResult,
+    );
+    if (isThreadMentionableResult) {
+      allowedToMentionThreadIDs.add(mentionedThreadID);
+    }
+  }
+  return allowedToMentionThreadIDs;
+}
+
 async function sendRescindNotifs(rescindInfo: PushInfo) {
   if (Object.keys(rescindInfo).length === 0) {
     return;