/**
 * MX lookup cache to avoid redundant DNS lookups in batch processing.
 */
export declare class MxCache {
    private cache;
    private ttlMs;
    constructor(ttlSeconds?: number);
    get(domain: string): {
        hasMx: boolean;
        blanketReject?: boolean;
    } | null;
    set(domain: string, hasMx: boolean, blanketReject?: boolean): void;
    clear(): void;
    get size(): number;
}