* Callback method for outgoing {@link Message} that failed to be sent.
* Returns {@code true} if the {@link Throwable} was handled or not.
*/
public boolean handleException(Endpoint endpoint,
Message message, Throwable t) {
MessageId messageId = message.getMessageId();
MessageEntity entity = entityManager.get(messageId);
if (entity != null) {
entity.handleException(t);
return true;