/**
 * Delivery status for outbound email messages sent through the Email Gateway
 * to external recipients via SMTP.
 *
 * @remarks
 * - Queued: Message enqueued in the outbound queue, awaiting delivery attempt
 * - Sending: Delivery attempt in progress (handed to Postfix)
 * - Delivered: Postfix confirmed successful SMTP delivery
 * - TemporaryFailure: SMTP 4xx response; message will be retried with exponential back-off
 * - PermanentFailure: SMTP 5xx response or retries exhausted; DSN generated to sender
 * - Expired: Maximum retry duration exceeded without successful delivery
 *
 * @see Requirements 11.4, 3.1, 3.4, 3.5, 7.2
 */
export declare enum OutboundDeliveryStatus {
    Queued = "queued",
    Sending = "sending",
    Delivered = "delivered",
    TemporaryFailure = "temporary_failure",
    PermanentFailure = "permanent_failure",
    Expired = "expired"
}
/**
 * Valid state transitions for the OutboundDeliveryStatus state machine.
 *
 * Transitions:
 * - Queued → Sending
 * - Sending → Delivered, TemporaryFailure, PermanentFailure
 * - TemporaryFailure → Queued, PermanentFailure, Expired
 * - Delivered → (terminal)
 * - PermanentFailure → (terminal)
 * - Expired → (terminal)
 *
 * @see Requirements 3.1, 3.4, 3.5
 */
export declare const OUTBOUND_VALID_TRANSITIONS: Record<OutboundDeliveryStatus, OutboundDeliveryStatus[]>;
/**
 * Validates whether a transition from one OutboundDeliveryStatus to another
 * is allowed by the state machine.
 *
 * @param from - The current outbound delivery status
 * @param to - The target outbound delivery status
 * @returns true if the transition is valid, false otherwise
 *
 * @see Requirements 3.1, 3.4, 3.5
 */
export declare function validateOutboundTransition(from: OutboundDeliveryStatus, to: OutboundDeliveryStatus): boolean;
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