incoming email did not generate new issue

I have created a service to create issues from emails generated to a specific address. In all my testing it has succeeded every time. But today, one of the incoming emails (which I verified was sent by checking that email address' mail on the server).
In 15 tests, this is the only one that failed. But it is crucial we don't have any fall off into cyberspace because we are using this method to create internal requests via e-mail. So, if an email doesn't generate an email, that is very bad. Is there some error log that we can look at to see if this has happened?

Note: the email is generated programmatically by a user checking a box on the form, so there is no chance of the email address being entered in the wrong case, because it's worked before.

Thanks