/**
 * MailboxEventEmitter — minimal pub/sub for real-time push to SSE clients.
 *
 * The HTTP bridge uses this to push `send`/`ack`/`delete` events to
 * connected SSE clients without requiring them to poll. Each event is a
 * shallow copy of the relevant data (message id, action, timestamp) —
 * never the full mailbox state.
 *
 * The emitter is intentionally simple: a Set of listeners, add/remove,
 * and emit. No buffering, no replay — if a client connects after a send,
 * it won't receive past events (it should query first to catch up).
 *
 * @module mailbox-events
 */
export type MailboxEventType = 'message.sent' | 'message.acked' | 'message.deleted' | 'message.restored';
export interface MailboxEvent {
    type: MailboxEventType;
    messageId: string;
    from?: string | undefined;
    to?: string | undefined;
    timestamp: string;
}
export type MailboxEventListener = (event: MailboxEvent) => void;
export declare class MailboxEventEmitter {
    private listeners;
    subscribe(fn: MailboxEventListener): () => void;
    emit(event: MailboxEvent): void;
    /** Number of active subscribers (for observability / metrics). */
    get subscriberCount(): number;
    clear(): void;
}
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