import type { MailOptions, PreviewConfig, SendResult, Transport, VerifyResult } from "../core/types.js";
/**
 * Development transport that writes emails to disk instead of sending them.
 */
export declare class PreviewTransport implements Transport {
    readonly provider = "preview";
    /** Directory where preview `.eml` or `.html` files are written. */
    private readonly outDir;
    /** Whether to open the written file in the default browser. */
    private readonly open;
    /** Output format: full MIME (`.eml`) or HTML body only. */
    private readonly format;
    /** Creates a preview transport with optional output directory and format. */
    constructor(config?: PreviewConfig);
    /** Writes the message to disk and returns a synthetic SendResult. */
    send(options: MailOptions): Promise<SendResult>;
    /** Always succeeds — preview transport requires no external connectivity. */
    verify(): Promise<VerifyResult>;
}