/**
 * SMTP Client Validation Utilities
 * Input validation functions for SMTP client operations
 */
import type { ISmtpClientOptions, ISmtpAuthOptions } from '../interfaces.js';
/**
 * Validate email address format
 * Supports RFC-compliant addresses including empty return paths for bounces
 */
export declare function validateEmailAddress(email: string): boolean;
/**
 * Validate SMTP client options
 */
export declare function validateClientOptions(options: ISmtpClientOptions): string[];
/**
 * Validate authentication options
 */
export declare function validateAuthOptions(auth: ISmtpAuthOptions): string[];
/**
 * Validate hostname format
 */
export declare function validateHostname(hostname: string): boolean;
/**
 * Validate port number
 */
export declare function validatePort(port: number): boolean;
/**
 * Sanitize and validate domain name for EHLO
 */
export declare function validateAndSanitizeDomain(domain: string): string;
/**
 * Validate recipient list
 */
export declare function validateRecipients(recipients: string | string[]): string[];
/**
 * Validate sender address
 */
export declare function validateSender(sender: string): boolean;