Up until yesterday the exchange server was running without any problems. Then we had a call from a customer saying that they had not received an email when we had sent it. I have since found that any email we send to an external address does not get receive. We can receive emails fine. We can even send emails internally. I have checked our firewall. I have looked through exchange and can't find anything obvious. I have not changed anything on the server that I can think might have caused this. The mail simply goes to the outbox like its being sent then a few seconds later moves to the sent items box. No error message get bounced back they simple disappear to nowhere. I have tested sending emails to multiple email domains so I don't think it's those I am sending to. I am not using smart host although I tried setting that up with no luck. Any suggestion would be grateful. This is rather urgent as we have not been able to send emails for 2 days now.