When you are sending out e-mail, are you using a valid e-mail address on the server to send out from?
If
example.com exists on your server and you are using
mail.example.com as your outgoing mail server and you are sending out mail from
[email protected]. Exim on your server (
mail.example.com) will see that the domain you are sending from (
example.com) exists on the server. It will then check to make sure that the e-mail address you are sending from (
[email protected]) exists on this server. If it does not then the transaction will fail, because the sender verification process will fail. If you do not have the
[email protected] e-mail address set up, then the default box will be checked. Setting your default box to
:blackhole: is the same as accepting the e-mail just not delivering it anywhere. So in this situation, any e-mail address on the
example.com domain will be seen as valid. When the default box is set to
:fail: only those addresses that are set up as actual mail account and/or forwarders will be accepted.
Basically, you need to insure that you are sending mail from a valid e-mail address that can also accept e-mail on the server.