It might be one of the simplest usage scenarios, but it is also one of the most useful: Queueing up work to be handled in a reliable fashion, with retries and error queues and everything :)
In this case, it's about sending emails. The SmtpClient is injected into SendEmailHandler,
and it might be badly configured (like e.g. using the wrong port on the SMTP server, missing
configuration, etc.), which is why it is good to have a message queue in front of the actual
sending of the mails.