cherry pick

packages/backend/src/core/NoteCreateService.ts

@@ -59,6 +59,7 @@ import { UtilityService } from '@/core/UtilityService.js';
 import { UserBlockingService } from '@/core/UserBlockingService.js';
 import { isReply } from '@/misc/is-reply.js';
 import { trackPromise } from '@/misc/promise-tracker.js';
+import { IdentifiableError } from '@/misc/identifiable-error.js';
 
 type NotificationType = 'reply' | 'renote' | 'quote' | 'mention';
 
@@ -365,6 +366,15 @@ export class NoteCreateService implements OnApplicationShutdown {
 			mentionedUsers.push(await this.usersRepository.findOneByOrFail({ id: data.reply!.userId }));
 		}
 
+		const willCauseNotification = mentionedUsers.filter(u => u.host === null).length > 0 || data.reply?.userHost === null || data.renote?.userHost === null;
+
+		if (process.env.MISSKEY_BLOCK_MENTIONS_FROM_UNFAMILIAR_REMOTE_USERS === 'true' && user.host !== null && willCauseNotification) {
+			const userEntity = await this.usersRepository.findOneBy({ id: user.id });
+			if ((userEntity?.followersCount ?? 0) === 0) {
+				throw new IdentifiableError('e11b3a16-f543-4885-8eb1-66cad131dbfd', 'Notes including mentions, replies, or renotes from remote users are not allowed until user has at least one local follower.');
+			}
+		}
+
 		if (data.visibility === 'specified') {
 			if (data.visibleUsers == null) throw new Error('invalid param');