/**
 * MIME/multipart header helpers shared across email connectors (Mailgun's
 * multipart body, mailbox/encoded-word formatting for SES, Resend, Postmark
 * and Mailgun). Attacker-influenced attachment `contentType`/`filename`
 * values must never be able to inject or override MIME/multipart part headers
 * via embedded CR/LF.
 */
/**
 * Strip CR/LF characters from a header value to prevent header injection.
 * MIME header field bodies cannot contain bare CR/LF; any caller-supplied
 * value that does is sanitised to spaces.
 */
export declare function stripCrlf(value: string): string;
/**
 * Sanitize a MIME/multipart filename WITHOUT adding surrounding quotes: strips
 * CR/LF then backslash-escapes embedded `"`/`\`. For templates that already
 * supply the surrounding `"..."` (e.g. Mailgun `filename="${...}"`). Use
 * `quoteMimeFilename` when the surrounding quotes are not already present.
 */
export declare function escapeMimeFilename(name: string): string;
/**
 * Quote a MIME filename that may contain `"` or other special characters per
 * RFC 2183. Bare filenames are quoted; embedded `"` is backslash-escaped.
 */
export declare function quoteMimeFilename(name: string): string;
/**
 * Format an RFC 5322 mailbox (`display-name <addr-spec>`) for vendors that
 * take `From` as one string (SES `FromEmailAddress`, Resend, Postmark,
 * Mailgun).
 *
 * - An atext-only name is sent as-is (`Acme Mail <a@b.c>`).
 * - An ASCII name containing specials (`, . " ( ) < > @ : ; [ ] \`) must be a
 *   quoted-string, or `Doe, Jane` would parse as two mailboxes.
 * - A non-ASCII name becomes RFC 2047 encoded-words and is NOT quoted: RFC
 *   2047 §5 says "An 'encoded-word' MUST NOT appear within a
 *   'quoted-string'". SES's developer guide requires this form for a Unicode
 *   "friendly from" name.
 */
export declare function formatMailbox(name: string | undefined, addr: string): string;
/**
 * RFC 2047 base64 encoded-words for a header value. ASCII input is returned
 * unchanged. RFC 2047 §2: "An 'encoded-word' may not be more than 75
 * characters long", so the UTF-8 bytes are split — only on character
 * boundaries — into 45-byte runs (60 base64 chars + 12 for `=?UTF-8?B?…?=`)
 * and the words are joined with a space, which decoders drop between
 * adjacent encoded-words.
 */
export declare function encodeWords(value: string): string;