Very often SPAMMERS take advantage of catch-all email setup on webservers. Every email no matter what the recipients email address is will be caught by the default email account. It is highly recommended not to use catch-all email accounts and to discard SPAM send to non-existing email addresses. SPAM will clog up your SMTP server and consum resources like bandwidth and disk space in mailboxes. In most mail servers and web control panels (like cPanel) the user or admin can decide what will happen to emails with no existing recipient on the server. Refuse to the let emails onto the server and to let the sender's mail server deal with it (option: ":fail:") or to accept these incoming messages but then to delete them right away (option: ":blackhole:".

Our conclusion:

:fail: is the preferred option over :blackhole:.

The :blackhole: option accepts everything sent to the domain mail server and then throws away the email going to a non-existing email account. This option uses the full amount of bandwidth, and also requires that the server be reading and writing messages to disk before they are deleted... multiply this by 1,000 messages a day or so and you can imagine the impact onto your server resources. Imagine your an email address from your domain (a non-existing one) has been spoofed to send out SPAM messages that even carry a virus. Most natural many of these messages will bounce back and hit your mail server. Your web server would have to deal with thousands of attachments at a time. Performance will go down - no matter what.

:fail: stops emails send to invalid recipients from entering the mail server in the first place..Exim will reject each message during the smtp handshake conversation - therefore the actual email message will never make it to your server. It is being refected from entering your system and the sender's email server has to deal with the stuck message. This option is also the better one for legitimate emails where the sender has actually mis-spelled the recipients email address. He would get a bounce message informing him so that the error can be corrected. Depending on your hardware - a server can handle many more :fails: than it can do :blackholes:.