/**
 * Masks the username part of an email address for privacy or anonymization.
 *
 * The function preserves the domain while replacing most characters in the username
 * with asterisks (`*`). It keeps the first two characters visible when the username
 * is longer than two characters. If the input is invalid or not a string containing
 * an `@` symbol, it returns the original value unchanged.
 *
 * @example
 * ```ts
 * maskEmail("john.doe@example.com");
 * // → "jo******@example.com"
 *
 * maskEmail("ab@example.com");
 * // → "ab@example.com"
 *
 * maskEmail("a@example.com");
 * // → "*@example.com"
 * ```
 *
 * @param email - The email address to mask.
 * @returns A masked version of the email, or the original value if invalid.
 */
export declare const maskEmail: (email: string) => string;
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