        /// <summary>
        /// Will clear the seen messages cache.
        /// </summary>
        /// <returns>A collection of message ids which have already passed.</returns>
        IEnumerable<string> PurgeSeenMessagesCache();
        /// <summary>
        /// Extracts the Envelope's header.
        /// </summary>
        /// <param name="envelope">The <see cref="TransportEnvelope"/> to retrieve the header from.</param>
        /// <returns>The <see cref="ITransportEnvelopeHeader"/> belonging to the given <see cref="TransportEnvelope"/>.</returns>
        ITransportEnvelopeHeader ExtractHeader(TransportEnvelope envelope);
        ///// <summary>