/// <summary>
        /// Posts a message.
        /// </summary>
        /// <param name="contactId">Identifier of the contact.</param>
        /// <param name="messageBuilder">Message builder.</param>
        public void PostMessage(Guid contactId, INotificationMessageBuilder messageBuilder)
        {
            messageBuilder.CheckArgumentNull("messageBuilder");
            Guid adminUnitId = FindSysAdminUnitId(contactId);

            messageBuilder.SysAdminUnitId = adminUnitId;
            IMsg        message = messageBuilder.Build();
            IMsgChannel channel = _channelManager.FindItemByUId(adminUnitId);

            if (channel != null)
            {
                channel.PostMessage(message);
            }
        }