What to do when your customers emails are blocked

http://blogs.technet.com/sbs/archive/2008/01/03/what-to-check-when-exchange-cannot-send-email-to-certain-domains.aspx

This is a neat post that describes why your customers might call you and say 'so and so didn't get my email and I sent 50 test ones!'....

The majority of the time, we find that the email issues appear on the remote/recipients server. In the case of our customers we almost always use a smarthost for outbound mail (we like dyndns.com). By setting up an account with DynDns, we get an additional bonus: A backup mail server that will hold our customers incoming mail in case the server isn't available.