Why is my Default Address in cpanel set to ":fail: No such user here"?

Your Default Email Address is where email is sent when the message is not addressed to any established mailbox.

The setting ":fail: no such address here" sends a message back to the spammer that the address they sent to does not exist.

Keep in mind that most spammers will know your domain name, but they may not know your specific email addresses. In order to get their message to you, they'll send spam to five, ten or more "best guess" addresses such as [e-mail], [e-mail], [e-mail], [e-mail], [e-mail] and so on, hoping that at least one will get through to you.

You really don't want all these junk mail messages, do you?

If you remove the ":fail: no such address here" and replace it with a legitimate address, then ALL of this "best guess" spam will get through to you and your mailbox will become very crowded very quickly.

Forwarding messages in this manner is called a "catch all" email address and it is a VERY BAD IDEA. Using a "catch all" email address only encourages more and more spam and it puts an extra burden on our server to process those unwanted messages.

For this reason we recommend that our members create either mailboxes or forwarders for each email address they plan to use and never rely on the default address.

See more here:
What is the default address used for in cPanel?
How can I create and use an email forwarder?