User will have question while configure the setting for unrouted e-mail:
-Discard (means :blackhole)
-No Such User Here (mean :fail)
-forward to catch all account
Below is the explanation for the setting on :fail vs blackhole
:blackhole: email is accepted and received into the server in its entirety. It is then processed through exim and only on delivery is it written to the null device (/dev/null) and silently ignored. This option consumes server resources because processing is needed, and waste bandwidth, etc.
:fail: the email is never accepted into the server. During the initial SMTP negotiation when the senders SMTP server connects to your SMTP server, the sending SMTP server issues a RCPT command notifying your server which email address the email to follow is intended for. Your server then checks whether the recipient email actually exists on your server (a POP3 account, an alias or a catchall alias) and if it does not, it issues an SMTP DENY which terminates the attempt to deliver the email. This option saves server resources, bandwidth, etc
For shared environment, I would recommend that not to define catch-all account ad it will overload the server, receive many spam emails, consume resources like bandwidth and disk space in mailboxes.
