/**
 * RFC 2822 message construction for the Gmail API.
 *
 * Used by gmail_drafts_create. Construction is deliberately minimal — no
 * attachments, single text/plain or text/html body, optional multipart/alternative
 * when both are provided. Output is plain RFC 2822 text; callers are expected
 * to base64url-encode it before handing to Gmail's `raw` field.
 */
export interface MimeOptions {
    /** Recipient(s). Comma-separated for multiple, e.g. "a@x.com, b@y.com" */
    to: string;
    cc?: string;
    bcc?: string;
    subject?: string;
    /** Plain-text body */
    body?: string;
    /** HTML body. If both body and htmlBody are set, output is multipart/alternative. */
    htmlBody?: string;
    /** Message-ID of the message being replied to (improves threading). */
    inReplyTo?: string;
    /** Space-separated Message-IDs of ancestor messages. */
    references?: string;
    /** Optional explicit From; usually omitted so Gmail fills in the authenticated user. */
    from?: string;
    /** Override the multipart boundary for deterministic test output. */
    boundary?: string;
}
/** Encode a UTF-8 string to base64url (RFC 4648 §5). */
export declare function base64url(input: string): string;
/**
 * Encode a header value containing non-ASCII characters per RFC 2047 (B-encoding).
 * Pure-ASCII values pass through unchanged so common headers stay readable.
 */
export declare function encodeHeader(value: string): string;
/** Build an RFC 2822 message string. */
export declare function buildRfc2822(opts: MimeOptions): string;