This controls much more that you'd expect., your "default email address" is the one, which can be used as a "catch all", or in other words, to "catch all mail", which is addressed to firstname.lastname@example.org. Using a catch all email is mostly a curse and will bring you tons of spam. We advise all our users to disable this function. Spammers or junk mailers use this feature as a means to pump their trash into your mailbox. As long as the "catch all" is enabled, then all they must do is send to email@example.com and it will reach you.
We recommend that you DO NOT USE the catch all
Setting your default email account:
1. Login to C-Panel
2. Select "Default Address"
3. Select "Discard with error to sender (at SMTP time) "
Select "Change" and you'll see a confirmation box, which displays your new setting. That's it- done!
What happens now?
When a spammer or junk mailer attempts to use a random email address to Spam you, it will be bounced back to them. That is, unless they happen to get a hold of one of your "legitimate pop email account names", in which case, you'd have a different problem on your hands.
In the 'Advanced' option is a setting 'Discard' and we do not recommend that. Mail is much better handled with the above setting.
See also How To Combat Spam